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Acetaminophen – Partisan Politics Corrupts Scientific Assessments – Part 2

July 30th, 2026

Most medico-legal controversies over causation resolve in the pages of medical journals and within the walls of state and federal courthouses. The claim that pregnant women’s ingestion of acetaminophen causes their children to have an increased risk of autism, however, has become a flash point in American political drama.[1]

After the district court entered judgment against plaintiffs in the acetaminophen MDL, a sea change in the politicization of science occurred in the executive branch of the United States. In November 2024, the United States returned Donald Trump, its first fully post-modern president to office. For most of his life, Trump, in an homage to Michel Foucault, has seen no difference between assertions and truth. In the run up to the general election of 2024, Trump forged an alliance with independent candidate Robert F. Kennedy, Jr., whose disdain for evidence and truth has long made him a notorious source of misinformation on vaccines, autism, antidepressants, pesticides, glyphosate, among other things.

In August 2024, Kennedy quit his campaign and endorsed Trump, in return for a role in a future Trump Administration. After promising Kennedy a cabinet level position, Trump told his supporters that: “I’m going to let him go wild on health. I’m going to let him go wild on the food. I’m going to let him go wild on the medicines.”[2] Trump kept his promise of an appointment to Kennedy. On February 13, 2025, Robert F. Kennedy Jr. was confirmed by a narrow vote of the Senate, and sworn in as Secretary of Health and Human Services. President Trump immediately issued Executive Order 14211 to establish a “Make America Healthy Again [MAHA] Commission,” to be chaired by Secretary Kennedy.[3] One of the key objectives of the MAHA commission included investigating the causes of chronic childhood diseases and “assess[ing] the prevalence of and threat posed by the prescription of selective serotonin reuptake inhibitors, antipsychotics, mood stabilizers, stimulants, and weight-loss drugs.”

Two months later, at a press conference in April 2025, Kennedy asserted that environmental toxic substances were responsible for the increasing prevalence of autism. Only a day earlier, the Centers for Disease Control issued a report in which over 50 government scientists explained that evolving diagnostic criteria and improved awareness of parents and teachers (not to mention entitlements to social and educational support) were largely responsible for the changing patterns of diagnosis and assessment.[4]

Channeling his lawsuit industry practice in which Kennedy labeled any adverse evidence as a manufacturing industry conspiracy, the Secretary dismissed the CDC’s explanations as a (manufacturing) industry “canard” to deflect attention from possible environmental causes. Kennedy promised a massive research initiative to identify the environmental “culprit,” with results to be available by September 2025.[5]

The scientific community might well have been skeptical about how Kennedy was going to deliver answers to etiological questions surrounding autism in five months, when causal conclusions had eluded the scientific world for decades. In the fullness of the next five months, not much new research was done, but with the clock ticking, a conclusion was launched in the absence of evidence.

On September 22, 2025, President Trump, along with Secretary Kennedy, FDA Commissioner Marty Makary, Director of the National Institutes of Health Jay Bhattacharya, and Administrator of the Centers for Medicare & Medicaid Services Mehmet Oz, called a news conference to announce the “conclusion.”[6] Stumbling over the pronunciation of acetaminophen, Trump told women not to take the medication; it caused autism. Kennedy struck a more moderate note about acetaminophen, in describing a “potential” association between acetaminophen and ASD and ADHD. He acknowledged reviewing “contrary” studies showing no association, and announced that the FDA would issue a notice to physicians and begin a change in the medication label.

When Commissioner Makary spoke, he cited studies showing “consistent” associations between acetaminophen and autism. In a flourish, Makary stated that:

“four weeks ago, a Mount Sinai Harvard study [“Pada 2025”] reviewed all the existing literature and found the overwhelming body of evidence points to an association. Sure, you’ll be able to find a study to the contrary, that’s how science works, but to quote the dean of the Harvard School of Public Health, there is a causal relationship between prenatal acetaminophen use and neurodevelopmental disorders of ADHD and autism spectrum disorder.”[7]

The dean referenced by Makary was none other than Andrea Baccarelli, the key expert witness for plaintiffs’ in the acetaminophen MDL, who was excluded by Judge Denise Cote’s Rule 702 decision. Baccarelli has only recently moved from Columbia University to assume a new position at Harvard. The Mount Sinai study published four weeks earlier was a review by four authors, including Baccarelli.[8] Baccarelli and his co-authors, however, decidedly did not embrace a causal conclusion in their publication. In their statement of conclusions in the paper’s abstract, Baccarelli and his co-authors stated that their “analyses using the Navigation Guide thus support evidence consistent with an association between acetaminophen exposure during pregnancy and increased incidence of NDDs.” (emphasis added). In the discussion section of their paper, the authors explained that their “analysis demonstrated evidence consistent with an association between exposure to acetaminophen during pregnancy and offspring with NDDs, including ASD and ADHD, though observational limitations preclude definitive causation.”[9] In his press conference remarks, Makary was quoting not from a peer-reviewed article, but from Baccarelli’s expert witness report, written for litigation, and excluded from evidence by Judge Cote’s decision in 2023.

Makary’s reference to language from Baccarelli’s 2023 expert witness litigation report thus raises two interesting questions. First, given that expert witness reports do not generally find their way to the desks of the FDA Commissioner, how did Makary come by this plaintiffs’ litigation report? Second, and more troubling, why did Baccarelli defend his causal conclusion under oath, at an examination before trial in 2023,[10] when he was unwilling to embrace a causal conclusion two years later, in 2025? The first question raises at least the appearance of collusion between the lawsuit industry and political actors such as Secretary Kennedy and Commissioner Makary. The second question raises the issue of fraud on the court by a witness who is willing to assert a causal conclusion in legal proceedings, but is not willing to publish and defend the causal conclusion to his peers in a scientific journal.

In the wake of the news conference, the New York Times talked with Baccarelli, and reported that he opined in the interview that “[f]urther research is needed to confirm the association and determine causality, but based on existing evidence, I believe that caution about acetaminophen use during pregnancy — especially heavy or prolonged use — is warranted.”[11] In essence, Baccarelli was offering a precautionary principle interpretation of the available evidence, not a causal conclusion. Baccarelli issued a statement on social media, the day of the news conference, to state that he and his colleagues “had conducted a rigorous review funded by a grant from the National Institutes of Health (NIH), of the potential risks of acetaminophen use during pregnancy.” Baccarelli disclaimed having reached a causal conclusion, describing his review as showing merely “evidence of an association between exposure to acetaminophen during pregnancy and increased incidence of neurodevelopmental disorders in children,” and that “[f]urther research is needed to confirm the association and determine causality….”[12] As we will see, Baccarelli’s claim that the Prada review was funded by an NIH grant was a bit of a tall tale, but his disclaiming a causal conclusion and explaining that his position was based upon the precautionary principle, not a showing of medical causation, was both candid and at odds with his litigation expert report.

The lead author of the Prada Review, Diddier Prada, an epidemiologist at Mount Sinai Hospital in New York, similarly told the Times that “[w]e cannot answer the question about causation — that is very important to clarify.”[13] A third author of the Prada Review was Ann Bauer, the lead author of the 2021 putative consensus paper that was instrumental in starting the acetaminophen mass tort. Bauer commented in advance of the September press conference. In anticipating the Administration’s embrace of a causal claim, Bauer noted that she was “a little concerned about how this message is going to come because I think they may be jumping the gun”; “I think those of us in the research community would like to see stronger evidence.”[14]

Scientific Community’s Reaction to Trump’s News Conference

Trump’s press conference on acetaminophen and vaccines was widely met with condemnation and disbelief.[15] Most medical organizations were appalled at the administration’s sloppy messaging. The day after the press conference, the American College of Medical Toxicology (ACMT) issued a statement in response, to affirm the safety of acetaminophen in pregnancy. The ACMT noted that its position agreed with those of the American College of Obstetrics and Gynecologists, the Society for Maternal-Fetal Medicine, the American Academy of Pediatrics, and the Society for Developmental and Behavioral Pediatrics.[16]

In Europe, where pharmaceutical regulation is typically even more precautionary than in the United States, the European Medicines Agency (EMA) announced, a day after the Trump news conference, that using acetaminophen during pregnancy was safe, with no showing that it causes autism in offspring. The EMA announcement quoted Steffen Thirstrup, the EMA’s Chief Medical Officer, as stating that “[p]aracetamol remains an important option to treat pain or fever in pregnant women. Our advice is based on a rigorous assessment of the available scientific data and we have found no evidence that taking paracetamol during pregnancy causes autism in children.”[17] In the United Kingdom, the governmental Medicines & Healthcare Products Regulatory Agency issued a notice, on September 23, 2025, that acetaminophen was safe to take in pregnancy and that the medication has not been shown to cause autism in children.[18]

The day of the news conference, the Autism Science Foundation issued a statement of deep concern about Trump’s announcement. The Foundation observed that “[b]ased on the existing data, there is not sufficient evidence to support a link between acetaminophen and autism.”[19] Medical ethicist Dr. Professor Arthur Caplan, who is not given to hyperbole, called the conference “the saddest display of a lack of evidence, rumors, recycling old myths, lousy advice, outright lies, and dangerous advice I have ever witnessed by anyone in authority.”[20]

The official governmental messaging in the wake of Trump’s press conference was more circumspect. In her press release on the day of the news conference, Press Secretary Karoline Leavitt used the meaningless locution, “suggested link,” and cited the Prada review, which eschewed causal conclusions.[21] Leavitt gratuitously added that “[t]he Trump Administration does not believe popping more pills is always the answer for better health.” Unless of course, it is Propecia for Donald Trump, testosterone for Peter Hegseth, or ketamine for Elon Musk.

When the administration physicians communicated with the public, they said something very different from Trump’s presentation. Dr. Mehmet Oz, former surgeon and television talking head, who stood mute by Trump’s side, found his voice later in the week, when he acknowledged that pregnant women of course should take acetaminophen when physicians direct them to do so.[22]

When FDA Commissioner Makary issued a notice to physicians, the same day as the Trump news conference, he carefully avoided causal language:

“In recent years, evidence has accumulated suggesting that the use of acetaminophen by pregnant women may be associated with an increased risk of neurological conditions such as autism and ADHD in children.

* * *

To be clear, while an association between acetaminophen and autism has been described in many studies, a causal relationship has not been established and there are contrary studies in the scientific literature.”[23]

An FDA news release, also on September 22, 2025, announced that the agency was beginning a change in safety labeling, but the agency press release spoke only in terms of association and correlation, and like the FDA notice to physicians, explicitly disavowed a causal conclusion.[24] The FDA also noted that it was undertaking a safety label change, but the official communication did not reveal the dodgy origins of this initiative. The impetus for the label change appears to have come not from FDA scientists, but from Aaron Siri, a plaintiff’s lawyer who worked in concert with Robert Kennedy for years in anti-vaccination litigation. In a remarkable coincidence, Siri filed a citizen’s petition with the FDA, the same day of Trump’s news conference to raise “exigent concerns” over a need for a label change.[25] The New York Times noted some of the troubling ethical concerns raised by Siri’s parallel activities. In 2023, Siri’s law firm recruited plaintiffs for acetaminophen mass tort litigation. In 2024, Siri represented Kennedy in his election campaign.[26] The week before the Trump news conference, Jill Schoff, an employee of Siri’s law firm, uploaded Baccarelli’s expert witness report from the acetaminophen MDL to the Internet Archive, which explains how Dr. Makary was able to quote from its conclusions at the September 22nd news conference.[27] Siri cited to Baccarelli’s for-litigation report, at the Internet Archive, in his citizen’s petition that plumped for a label change.

The politicization of the acetaminophen issue appeared to be complete, when a month after Trump’s press conference, Trump sycophant and Texas Attorney General Ken Paxton sued manufacturers of acetaminophen for “deceptively marketing Tylenol to pregnant mothers despite knowing that early exposure to acetaminophen, Tylenol’s only active ingredient, leads to a significantly increased risk of autism and other disorders.”[28] Only a month earlier, the FDA acknowledged in its press release and its notice to physicians, that causation was not known. Paxton filed the complaint in Panola County, Texas, where Trump received 81.4% of the counted votes in 2024.[29] In addition to lawyers at the Office of the Texas Attorney General, Ashley Keller and others from the Keller Postman LLC firm were listed as representing Texas in its epistemic and scientific overclaiming. Ashley Keller was the principal lawyer for plaintiffs in the MDL, which had been dismissed by Judge Cote in 2024.

(to be continued)

[1] I am indebted to, and thank, Barbara McIntyre for sharing some of the research for this series.

[2] Brett Samuels, Trump says he’ll let RFK Jr. ‘go wild’ on health and food in potential second term, THE HILL (Oct. 27, 2024), available at https://thehill.com/homenews/campaign/4956319-trump-says-hell-let-rfk-jr-go-wild-on-health-and-food-in-potential-second-term/.

[3] Executive Order 14211, Establishing the President’s Make America Healthy Again Commission”. The White House (Feb. 13, 2025), available at https://www.whitehouse.gov/presidential-actions/2025/02/establishing-the-presidents-make-america-healthy-again-commission/.

[4] Kelly A. Shaw, et al., Prevalence and Early Identification of Autism Spectrum Disorder Among Children Aged 4 and 8 Years — Autism and Developmental Disabilities Monitoring Network, 16 Sites, United States, 2022, 74 MORBIDITY & MORTALITY WEEKLY REP. 1 (Apr. 17, 2025).

[5] Meredith Wadman, Claiming autism ‘epidemic,’ RFK Jr. describes NIH initiative to find environmental causes, SCIENCE (Apr. 16, 2025), available at https://www.science.org/content/article/claiming-autism-epidemic-rfk-jr-describes-nih-initiative-find-environmental-causes

[6] Transcript, News Conference – Remarks: Donald Trump Makes an Autism Announcement at the White House, ROLLCALL (Sept. 22, 2025), available at

https://rollcall.com/factbase/trump/transcript/donald-trump-remarks-health-autism-white-house-september-22-2025/ (Transcript).

[7] Transcript at 24:35.

[8] Diddier Prada, Beate Ritz, Ann Z. Bauer and Andrea A. Baccarelli, “Evaluation of the evidence on acetaminophen use and neurodevelopmental disorders using the Navigation Guide methodology,” 24 ENVT’L HEALTH 56 (2025) [Prada]

[9] Prada Review at 38.

[10] Notes of Testimony from Deposition of Andrea Baccarelli, In re Acetaminophen ASD-ADHD Prods. Liab. Litig., Case No. 1:22-md-03043-DLC (Aug. 14, 2023).

[11] Azeen Ghorayshi, Trump Issues Warning Based on Unproven Link Between Tylenol and Autism, N.Y. TIMES (Sept. 22, 2025), available at https://www.nytimes.com/2025/09/22/health/kennedy-autism-tylenol-trump.html?searchResultPosition=2

[12] Statement from Andrea Baccarelli on X (Sept. 22, 2025), available at https://x.com/RapidResponse47/status/197023187271541578.

[13] Id.

[14] Jon Hamilton, Yuki Noguchi & Nell Greenfieldboyce, Trump blames Tylenol for autism. Science doesn’t back him up, NAT. PUBLIC RADIO (Sept. 22, 2025) (quoting Dr. Bauer), available at https://www.npr.org/sections/shots-health-news/2025/09/22/nx-s1-5550153/trump-rfk-autism- tylenol-leucovorin- pregnancy#:~:text=Many%20physicians%20and%20autism%20advocacy,studies%20in%20the %20scientific%20literature.%22

[15] Jeff Mason, Ahmed Aboulenein, and Julie Steenhuysen, Trump Links Autism to Tylenol and Vaccines, Claims Not Backed by Science, REUTERS (Sept. 22, 2025) (quoting Britain’s health regulator as saying that using paracetamol was safe and that there is no evidence paracetamol caused autism in children; quoting Coalition of Autism Scientists as stating that “[t]he data cited do not support the claim that Tylenol causes autism … .”) available at https://www.reuters.com/business/healthcare-pharmaceuticals/trump-expected-link-autism-with-tylenol-experts-say-more-research-needed-2025-09-22/; Brianna Abbott & Andrea Petersen, The Trump administration said acetaminophen could cause autism. Doctors maintain it is safe during pregnancy, WALL ST. J. (Sept. 22, 2025) (“Doctors and medical societies say acetaminophen, the active ingredient in Tylenol, remains the best option for treating fever and pain during pregnancy”), available at https://www.wsj.com/politics/policy/tylenol-pregnancy-autism-trump-report-what-to-know-2873d8e1; Will Weissert, Dr. Trump? The president reprises his COVID era, this time sharing unproven medical advice on autism, ASSOC. PRESS NEWS (Sept. 23, 2025), available at https://apnews.com/article/trump-doctor-vaccines-autism-tylenol-covid-disinfectants-22f5bcfe2fd3c18fdd412419941541f8; Editorial Board, ‘I’m not a doctor’: This shoddy autism science helps no one; President Donald Trump’s rant on vaccines and Tylenol has no basis in fact, WASH. POST (Sept. 22, 2025), available at https://www.washingtonpost.com/opinions/2025/09/22/trump-autism-vaccines-tylenol-acetaminophen/; Vivian Ho & Karla Adam, WHO joins global health leaders rejecting U.S. acetaminophen warnings; The World Health Organization is the latest to criticize claims from President Donald Trump and Robert F. Kennedy Jr. linking autism to Tylenol use during pregnancy, WASH. POST (Sept. 24, 2025), available at https://www.washingtonpost.com/health/2025/09/24/paracetamol-acetaminophen-tylenol-autism-who-trump/; Ariana Eunjung Cha, Matt Viser & Dan Diamond, As Trump ties Tylenol to autism, doctors raise alarms; U.S. officials will issue a warning label for pregnant individuals — a move that contradicts the guidance from the major medical societies, WASH. POST (Sept. 23, 2025), available at https://www.washingtonpost.com/health/2025/09/22/tylenol-autism-trump-announcement/

[16] American College of Medical Toxicology, ACMT Position Statement: ACMT Supports the Safe Use of Acetaminophen in Pregnancy (Sept. 23, 2025), available at https://www.acmt.net/news/acmt-position-statement-acmt-supports-the-safe-use-of-acetaminophen-in-pregnancy/

[17] European Medicines Agency, Use of paracetamol during pregnancy unchanged in the EU (Sept. 23, 2025), available at https://www.ema.europa.eu/en/news/use-paracetamol-during-pregnancy-unchanged-eu

[18] Medicines & Healthcare Products Regulatory Agency, MHRA confirms taking paracetamol during pregnancy remains safe and there is no evidence it causes autism in children (Sept. 23, 2025), available at https://www.gov.uk/government/news/mhra-confirms-taking-paracetamol-during-pregnancy-remains-safe-and-there-is-no-evidence-it-causes-autism-in-children.

[19] Autism Science Foundation, Statement on White House Announcement on Autism (Sept. 22, 2025), available at

https://autismsciencefoundation.org/press_releases/asf-statement-wh-briefing/.

[20] Ali Swenson & Lauran Neergaard, “Trump makes unfounded claims about Tylenol and repeats discredited link between vaccines and autism,” ASSOC. PRESS (Sept. 23, 2025) (quoting Prof. Caplan), available at https://apnews.com/article/tylenol-cause-autism-trump-kennedy-0847ee76eedecbd5e9baa6888b567d66

[21] Karoline Leavitt, “FACT: Evidence Suggests Link Between Acetaminophen, Autism,” WHITE HOUSE (Sept. 22, 2025), available at https://www.whitehouse.gov/releases/2025/09/fact-evidence-suggests-link-between-acetaminophen-autism/

[22] Sara Moniuszko & Céline Gounder, Dr. Oz says “of course” pregnant women should take Tylenol if a doctor says so (Sept. 26, 2025), available at https://www.cbsnews.com/news/dr-oz-says-of-course-pregnant-women-should-take-tylenol-if-a-doctor-says-so/

[23] Martin A. Makary, Notice to Physicians on the Use of Acetaminophen During Pregnancy, (Sept. 22, 2025) (emphasis added), available at https://www.fda.gov/media/188843/download?attachment=&utm_medium=email&utm_source=govdelivery

[24] FDA News Release, FDA Responds to Evidence of Possible Association Between Autism and Acetaminophen Use During Pregnancy (Sept. 22, 2025), available at https://www.fda.gov/news-events/press-announcements/fda-responds-evidence-possible-association-between-autism-and-acetaminophen-use-during-pregnancy.

[25] Informed Consent Action Network, Citizen Petition regarding safety-related labeling changes for the use of over-the-counter acetaminophen drug products during pregnancy, (Sept. 22, 2025), available at https://static01.nyt.com/newsgraphics/documenttools/88fe67f3f222ab88/66655cc8-full.pdf

[26] Christina Jewett, Kennedy’s Ties to Ally Leading Vaccine Lawsuits Raise Ethical Concerns, N.Y. TIMES (Oct. 3, 2025), available at https://www.nytimes.com/2025/10/03/health/kennedy-aaron-siri-vaccines-lawsuits.html

[27] Andrea Baccarelli, Rule 26 Witness Report, In re Acetaminophen ASD-ADHD Prods. Liab. Litig., Case No. 1:22-md-03043-DLC (Oct. 10, 2023), uploaded to Internet Archive on Sept. 16, 2025, available at https://archive.org/details/baccarelli-expert-report-acetaminophen-asd-adhd.

[28] Office of Texas Attorney General, Press Release: Attorney General Paxton Sues Big Pharma Manufacturers for Deceptively Marketing Tylenol to Pregnant Mothers Despite Known Dangers to Unborn Children (Oct. 28, 2025) (emphasis added), available at https://www.texasattorneygeneral.gov/news/releases/attorney-general-paxton-sues-big-pharma-manufacturers-deceptively-marketing-tylenol-pregnant-mothers.

[29] Panola County, TX Voting, available at https://www.bestplaces.net/voting/county/texas/panola.

Posted in Causation, Conflicts of Interest, Expert Witnesses, Rule 702, Scientific Evidence, Scientific Misconduct, Scientific Publishing | No Comments »

Acetaminophen – No Cure for Bad Jurisprudence – Part 1

July 28th, 2026

INTRODUCTION

Two weeks ago, the Second Circuit reversed the summary judgment entered by Judge Denise Cote, in the acetaminophen autism litigation. Judge Cote had excluded the plaintiffs’ expert witnesses on causation, under Rule 702, in a lengthy, careful opinion that found numerous deviations from the standard of scientific care by the challenged witnesses.[1] Rule 702 of the Federal Rules of Evidence cried out for application, especially subparts (b), (c), and (d):

(b) the testimony is based on sufficient facts or data;

(c) the testimony is the product of reliable principles and methods; and

(d) the expert’s opinion reflects a reliable application of the principles and methods to the facts of the case.

The Second Circuit, in an opinion by Judge Guido Calebresi, held that Judge Cote abused her discretion with respect to the most important of the plaintiffs’ witnesses.[2] Judge Calebresi’s opinion in turn failed to engage meaningfully with the science of causation, and abused both the facts and the law to reach its conclusions. Judge Calebresi cited opinions discredited by the 2023 amendment to Rule 702, and he failed to engage with the legal requirements of that amendment and controlling decisions of the Supreme Court.

The errors in Judge Calebresi’s opinion are symptomatic of a deep confusion between causation and precautionary policy. This confusion is prevalent in litigation and regulation, and it even infects what purport to be purely scientific determinations, such as the carcinogenicity determinations of the International Agency for Research on Cancer.[3] To appreciate the depth and extent of Judge Calebresi’s errors requires exploring the scientific record and an analysis of the appellate opinion’s errors and omissions.

BACKGROUND

Acetaminophen is a generic medication available for treating pain (analgesic) and fever (antipyretic), without a prescription in the United Sates. The chemical compound, N-acetyl-para-aminophenol (or APAP) was first prepared in 1878, and it was initially used in clinical practice in 1893. Acetaminophen did not become commercially available in the United States until the 1950s. Starting in 1960, acetaminophen became available to consumers as an over-the-counter medication.[4]

The 1970s marked the discovery of potential harms to both pregnant women and fetuses from mothers’ use of aspirin as an analgesic or antipyretic medication. In the 1980s, scientists suspected that aspirin may cause Reyes syndrome in children. Because untreated fevers in pregnant women can harm fetuses, including miscarriage and birth defects, fever presents a difficult clinical problem for women and their physicians.[5] Non-steroidal anti-inflammatory drugs (NSAIDs) are associated with miscarriage and birth defects, and opioid pain medications are associated with birth defects and maternal and neonatal addiction.[6] Clinicians have thus considered acetaminophen the preferred analgesic and antipyretic medication for pregnant women because of its consistently reassuring safety profile, and the absence of alternatives.

In December 2021, a group of advocacy scientists published a self-proclaimed consensus that precautionary action should be taken to restrict the use of acetaminophen by pregnant women.[7] The authors included advocacy scientist Shanna Swan, who was a protagonist on behalf of plaintiffs in notorious litigations involving silicone gel implants[8] and bendectin, and David Kriebel, a zealous proponent of the precautionary principle.[9] Swan and co-authors manifestly did not conclude that the available scientific evidence supported a causal inference between maternal acetaminophen use and their children’s autism. Instead, the authors asserted a precautionary policy approach that acetaminophen “be used by pregnant women cautiously at the lowest effective dose for the shortest possible time.”[10] Although the authors appropriately stopped short of causal conclusions, they incoherently argued that they had a “professional and social responsibility” to argue for precaution “even in the face of uncertainty, in light of the serious consequences of inaction.”[11] Given that they were addressing an uncertain and undetermined causal nexus, the authors had no warrant for suggesting that there would be any benefits from their recommendations or any untoward consequences from their having taken no action at all.

The pretense of the self-proclaimed consensus was dispelled by spirited dissent in response to the Call for Precautionary Action. One letter to the editor chided the precautionists for their misleading analysis.[12] Another letter highly critical of the faux consensus, was submitted by 50 or so signatories, including members of the Organization of Teratology Information Specialists, the Society of Birth Defects Research and Prevention, and Anthony R. Scialli, a past president of the Teratology Society, and founder and editor-in-chief of the journal Reproductive Toxicology.[13]

The American College of Obstetricians and Gynecologists (ACOG) issued a strongly worded rebuttal statement to highlight that the precautionists had not cited any clear evidence to show that acetaminophen use during any trimester causes fetal developmental issues.[14] ACOG reiterated its identification of acetaminophen as the only safe analgesic and antipyretic medication for pregnant women. ACOG further pointed out that despite the fear-mongering language of the Call for Precautionary Action, ACOG’s clinical guidance has long emphasized that medications in pregnancy should be used only when needed, in moderation, and in consultation with physicians. Indeed, the Food & Drug Administration already required that all over-the-counter medications intended for systemic absorption, including acetaminophen, be labeled: “If pregnant or breast-feeding, ask a health professional before use.”[15] At best, the precautionists had not managed to say anything important; at worst, they had stoked serious and unnecessary anxiety and concern among families and their physicians.

The precautionists also stoked the lawsuit industry. True to form, that industry was unfazed by the lack of certainty and the inability to infer causation even by the advocacy scientists who called for precautionary action. Within a few months of the 2021 opinion piece, the lawsuit industry started to file complaints, and the mass of litigation quickly led to consolidation and transfer to the Southern District of New York, for coordinated pre-trial procedures as a multi-district litigation (MDL).[16] A mass tort was born without scientific support for actual causation between the product and the harm, autism and other neurocognitive impairments.

Unsurprisingly, given the fragile and indeterminate scientific record on causation, the defense moved to exclude the plaintiffs’ causation expert witnesses, pursuant to Rule 702, at the close of discovery. Finding that the plaintiffs’ expert witnesses’ testimony failed to evidence a reliable application of epidemiologic method to reach the proffered conclusions, the district court excluded the challenged testimony,[17] and in August 2024, the district court dismissed over 500 cases in the MDL.[18]

(to be continued)

[1] In re Acetaminophen ASD-ADHD Prods. Liab. Litig., 707 F. Supp. 3d 309 (S.D.N.Y. 2023). Judge Cote addressed the admissibility of an additional expert witness in a subsequent opinion. In re Acetaminophen – ASD-ADHD Prods. Liab. Litig., 2024 WL 6 3874183 (S.D.N.Y. Aug. 20, 2024).

[2] Rutledge v. Walgreen Co., Nos. 24-916-cv(L), 24-1121(Con), 24-2360(Con); 24-2594-cv, Slip Op. (2^nd Cir. July 13, 2026), available at https://scholar.google.com/scholar_case?case=13067399726798054350&q=rutledge+v.+walgreen&hl=en&as_sdt=2006.

[3] See Nathan Schachtman, IARC’s Precautionary Science: How the WHO Cancer Research Agency Misinforms Regulation and Litigation, Wash. Legal Found. Monograph (2026), available at https://www.wlf.org/wp-content/uploads/2026/04/WLF-Precautionary-Science-monograph.pdf

[4] Laurie F. Prescott. Paracetamol: past, present, and future, 7 AM. J. THERAPEUTICS 143 (2000).

[5] See, e.g., Stephanie Antoun, Pierre Ellul, Hugo Peyre, Michelle Rosenzwajg, Pierre Gressens, David Klatzmann & Richard Delorme, Fever during pregnancy as a risk factor for neurodevelopmental disorders: results from a systematic review and meta-analysis, 12 MOLECULAR AUTISM 60 (2021).

[6] Rachela Smith & Jonathan S. Zipursky, Safety of acetaminophen use in pregnancy: review of existing evidence, 8 AM. J. OBSTET. & GYN. 101925 (2025).

[7] Ann Z. Bauer, Shanna H. Swan, David Kriebel, Zeyan Liew, Hugh S. Taylor, Carl- Gustaf Bornehag, Anderson M. Andrade, Jørn Olsen, Rigmor H. Jensen, Rod T. Mitchell, Niels E. Skakkebaek, Bernard Jégou & David M. Kristensen, Paracetamol use during pregnancy — a call for precautionary action, 17 NATURE REV. ENDOCRINOL. 757 (2021). [Call for Precautionary Action]

[8] See Judge Jack B. Weinstein, Preliminary Reflections on Administration of Complex Litigation, 2009 CARDOZO L. REV. de novo 1, 14 (2009) (describing plaintiffs’ expert witnesses in silicone litigation as “charlatans”; “[t]he breast implant litigation was largely based on a litigation fraud. … Claims—supported by medical charlatans—that enormous damages to women’s systems resulted could not be supported.”).

[9] See, e.g., David Kriebel, Joel Tickner, Paul Epstein, John Lemons, Richard Levins, Edward L. Loechler, Margaret Quinn, Ruthann Rudel, Ted Schettler & Michael Stoto, The Precautionary Principle in Environmental Science, 109 ENVT’L HEALTH PERSP. 871 (2001).

[10] Call for Precautionary Action at 764.

[11] Call for Precautionary Action at 763 (emphasis added).

[12] Joseph O’Sullivan, Alexandra E. Cairns, Elena Plesca, Rebecca S. Black, Charlotte Frise, Manu Vatish and Ana Sofia Cerdeira, Paracetamol use in pregnancy — neglecting context promotes misinterpretation, 18 NATURE REV. ENDOCRINOL. 385 (2022) (noting that the Swan review, in the absence of conclusive evidence, was propagating a scare that acetaminophen use in pregnancy was unsafe and should be unduly restricted).

[13] Sura Alwan, Elizabeth A. Conover, Lorrie Harris-Sagaribay, Steven H. Lamm, Sharon V. Lavigne, Shari I. Lusskin, Sarah G. Obican, Alfred N. Romeo, Anthony R. Scialli & Katherine L. Wisner, Paracetamol Use in Pregnancy—Caution Over Causal Inference from Available Data, 18 NATURE REV. ENDOCRINOL. 190 (2022).

[14] American College of Obstetricians and Gynecologists, Response to Consensus Statement on Paracetamol Use During Pregnancy (Sept. 29, 2021), available at https://www.acog.org/news/news-articles/2021/09/response-to-consensus-statement-on-paracetamol-use-during-pregnancy.

[15] 21 C.F.R. § 201.63(a). See also Pregnant or Nursing Women, 47 Fed. Reg. at 54,751.

[16] 28 U.S.C. § 1407

[17] In re Acetaminophen – ASD-ADHD Prods. Liab. Litig., 707 F. Supp. 3d 309 (S.D.N.Y. 2023) (Cote, J.). Additional plaintiffs designated one other expert witness, who succumbed to a later Rule 702 challenge. In re Acetaminophen – ASD-ADHD Prods. Liab. Litig., No. 22md3043 (DLC), 2024 WL 3357608 (S.D.N.Y. Jul. 10, 2024).

[18] In re Acetaminophen – ASD-ADHD Prods. Liab. Litig., No. 22md3043 (DLC), Order & Opinion (Aug. 20, 2024).

Posted in Causation, Rule 702, Scientific Evidence | No Comments »

The Recrudescence of Ferebee – Part Two

July 1st, 2026

In 2010, almost 30 years after Ferebee was decided, the Solicitor General cited the case in an amicus brief before the Supreme Court case, in Matrixx Iniatives, Inc. v. Siracusano. The case was a securities fraud class action, which was dismissed initially by the trial court on consideration of the defendant’s motion that the complaint failed to allege causation supported by statistically significant studies. The Supreme Court would go on unanimously to reject causation as a criterion for establishing a prima facie case of securities fraud, which made statistical significance irrelevant. Because the FDA could (and later did) require the company to recall its product upon a showing that material evidence suggested that there might be a possible causally induced harm, the Court held that the plaintiff class did not have to allege causation.[1] The harm to the shareholders came in the form of management’s bullish financial projections for a product that was later recalled for safety concerns, even if the recalled product never was shown to cause any harm. The Solicitor General’s amicus brief advanced the Ferebee case as an example of a causal relationship that could be established “through consideration of multiple factors independent of statistical significance.”[2] Although the government’s amicus brief correctly discerned that the causal connection between paraquat exposure and pulmonary fibrosis was established without analytical epidemiologic studies, and the necessary tools of statistical analysis for such studies, the brief mistakenly placed the allegations that Zycam caused anosmia in the same conceptual framework as paraquat. Unlike paraquat toxicity, millions of people used Zycam for relief from cold and flu symptoms, and the alleged harm, anosmia, commonly occurs in the aftermath of colds and flu. The Zycam personal injury claims fared poorly in litigation because of the dearth of supportive evidence that was appropriate to support causation, as opposed to materiality in securities law.[3]

The Ferebee case correctly observed that epidemiology was not necessary to establish the causal claim involving paraquat dermal exposure and lung toxicity and fibrosis. At the time that Mr. Ferebee sustained extensive paraquat exposure as a result of his governmental employer’s extreme negligence, the scientific community fully accepted that paraquat exposure, by ingestion, inhalation, or dermal exposure caused systemic toxicity and deleterious lung effects. This “general causation” had been established by case reports and case series, along with studies of paraquat’s metabolic fate and distribution in humans and non-human animals (including non-human primates), and assessment of mechanistic effects in cells and tissues of the target organs affected by paraquat when it became systemically distributed in the human body.

About the time of the Ferebee litigation, a textbook on agricultural chemicals described the toxic effects of dipyridyl compounds in humans, including paraquat:

“Human Toxicology Experience: A considerable amount of clinical experience has been reported in the literature with over 100 cases of illness and/or death. The chemical is unique in the sense that there is not only an acute toxicity syndrome but, in addition, it has the ability to produce a delayed fibroblastic response in the lungs. The latter is usually the principal mechanism of death.

For industrial workers, paraquat is not considered very dangerous. Inhalation hazard is extremely low due to the low vapor pressure of the chemical. Nevertheless, protective respiratory equipment should be used particularly when other atmospheric contamination might occur. * * * On no occasion should an applicator be allowed to walk through drifting spray.” [4]

This textbook cited studies that suggested that dermal and respiratory exposure to paraquat did not appear to be a hazard to field applicators, despite the demonstration of absorption, as long as precautions against overexposure are taken.[5] The premise of the textbook discussion, that appropriate, well-known safety measures and protective gear are employed, was an important part of its analysis.

This early textbook discussion also flagged delayed lung fibrosis as the main problem caused by all modes of paraquat exposure, including dermal absorption:

“Although the acute symptoms of paraquat intoxication are of concern and are dangerous, the principal problem relates to the unique delayed manifestations of this chemical’s ability to produce a fibroblastic change in the lung which begins a number of days after absorption. * * * Experiments then found that it was possible to induce respiratory failure as a result of both dermal and aerosol routes of absorption (Newhouse, 1978).”[6]

An early review by the World Health Organization also emphasized that paraquat exposure was not expected to pose a health risk as long as safe work practices are followed:

“Occupational exposure to paraquat does not pose a health risk if the recommendations for use are followed and there is adherence to safe working practices.

* * *

In the small number of reported cases of paraquat poisoning allegedly resulting from occupational exposure, the cause can be identified as one or a combination of a number of factors, viz contamination of the skin with concentrated products, use of inadequately diluted solutions, use of faulty equipment, misuse of equipment (e.g., blowing blocked spray jets) or failure to take action in the event of contamination of skin or clothing.”[7]

Cases of dermal exposure to undiluted paraquat (20%), especially when exposure involved dermal exposure to the scrotum, can produce serious systematic toxicity.[8]

Ferebee was not given an appropriate respirator even when exposed to intense atmospheric contamination. He was drenched in paraquat spray, and remained drenched for hours. The gross negligence of his employer ensured that there would not be many similar cases, and that the tools of analytical epidemiology would not be available.[9] Indeed, epidemiology was never involved in determining general causation of paraquat exposure and lung fibrosis. Given that the outcome of interest would likely occur only in the context of negligent or intentional over-exposure, epidemiology will never be available.

Revisiting the Ferebee decision and the unique facts of the case place the decision in a better perspective for judging how courts continue to cite the case. The facts of the case readily distinguish the case, the claimed harm, and the manner of showing causation, from the facts in cancer, birth defects, and other cases where epidemiology is essential. The principle of charity would require the frequently quoted language on expert witness admissibility to be taken as a statement of the appellate standard of review for the jury’s determination of medical causation. Most of the glib characterizations of the Ferebee turn out to be wrong on close inspection of the case.

a. Ferebee was not a precedent under the Federal Rules of Evidence

Chevron’s evidentiary arguments were posed under Maryland law. Neither Rule 702[10] nor Rule 703[11] was ever mentioned in the district court or the Court of Appeals decisions.

b. Ferebee does not support a false distinction between scientific and legal causation.

In a later Bendectin birth defects case, Richardson v. Richardson-Merrell, Inc., the Court of Appeals struggled to distinguish Ferebee and its holdings based upon the ample epidemiologic evidence involving Bendectin. The Court mischaracterized Ferebee as not pertinent because it was on the “frontier of current medical and epidemiological inquiry.”[12] The Richardson court was impressed by the 20 years of research on Bendectin, including multiple epidemiologic studies. Unfortunately, the court was apparently ignorant of the irrelevance of epidemiology to the Ferebee case, and the extensive research base for determining the lung toxicity of paraquat. This ignorance seems to have resulted from Judge Mikva’s generalizations and overstatements of the paucity of evidence in Ferebee.

c. Ferebee does not support the false distinction between scientific and legal certainty.

Courts and commentators have attempted to explain the result in Ferebee by invoking what is largely a false distinction between scientific and legal certainty (or sufficiency). The Ferebee decision itself provided the ammunition by asserting a distinction between the requirements of scientific and legal decision making:

“In a courtroom, the test for allowing a plaintiff to recover in a tort suit of this type is not scientific certainly but legal sufficiency.”[13]

This was a common approach in distinguishing Ferebee, in the Bendectin litigation,[14] but it was picked up and promulgated by scientists and legal commentators.[15] Invoking this alleged distinction has become a common rhetorical move to excuse inadequate or insufficient evidence to support an expert witness’s causation opinion in litigation. The generalization from the facts of Ferebee to all scientific and legal questions of causation was wrong from the inception, and citations to a single case, Ferebee, cannot make those generalizations true.[16]

d. Ferebee did not establish the irrelevance or the dispensability of epidemiologic evidence in cancer or birth defects cases.

The Ferebee case observed that “a cause-effect relationship need not be clearly established by animal or epidemiological studies before a doctor can testify that in his opinion such a relationship exists.”[17] The bit about animal studies certainly cannot be part of the holding because the plaintiffs’ expert witnesses relied extensively on animal studies, along with human case reports, and human clinical studies of the metabolic fate and distribution of paraquat in both animals and humans.

The Ferebee case does properly stand for the proposition that there is a subset of all health effect cases for which epidemiologic evidence is unavailable and unnecessary for an expert witness to have for a valid conclusion of general and specific causation. The case illustrates how the ill effects of paraquat were observed shortly after exposure, and were sufficiently unique to not have a meaningful base- or background- rate. Adding the studies of absorption, metabolic fate and distribution, and mechanism of action, the plaintiff’s expert witnesses had an ample scientific, and legal, basis to assess causality.

The Ferebee case is sometimes mistakenly thought of as a cancer case, which would have required epidemiologic evidence.[18] This mistake may well result from later courts citing Ferebee in cancer cases, for the proposition that epidemiologic evidence is unnecessary to support plaintiff’s causal claim between some exposure and some cancer. Perhaps the Illinois Supreme Court has provided the most egregious example of this sort of mistake. In Donaldson, a case involving plaintiff’s exposure to coal tar and his later development of neuroblastoma, the Court confusedly found Ferebee to be “of particular significance.”[19] The Donaldson court affirmed the trial court’s admission of plaintiffs’ expert witness testimony about “a causal link causal link despite the lack of a statistical number of others with neuroblastoma and a history of coal tar exposure.”[20] The court rambled on about how the plaintiffs were not required to prove general or specific causation “with 100% certainty that neuroblastoma.” This assertion was a common strawman argument that channels a misreading of Ferebee. The defense in Donaldson did not argue that 100% certainty was required, and such a level of posterior probability has never been required in law or in science.

The Ferebee case also did not establish that statistical significance was not required in epidemiologic studies for expert witnesses to be able reasonably to rely upon such studies. Because epidemiologic evidence was not at issue, there was no holding about epidemiologic studies or statistical significance as a criterion of the validity of such studies.

e. Ferebee did not establish that a clinician can opine about causation without sufficient facts and data.

One of plaintiff’s expert witnesses in Ferebee was Dr. Crystal, who was both a physician and a research scientist. His opinion as an expert witness was hardly without supporting facts and data, and the facts and data were of the exact kind that led to the scientific acceptance of the causal connection between some paraquat exposures and lung fibrosis. Crystal’s opinion was certainly not proffered without any evidentiary basis, as some have suggested.[21] Nor was Ferebee a case in which expert witnesses opined without facts and data to support unprecedented opinions on general and specific causation.[22]

This overwrought, over-extended interpretation of Ferebee as permitting causation opinions based upon only clinical observations of the patient appeared in the first edition of the Reference Manual on Scientific Evidence, but disappeared in all subsequent editions. In the chapter by evidence law professor Margaret Berger, the Manual reported that Ferebee was frequently cited for a “holding that causation can be established by the testimony of treating physicians.”[23] Berger’s observation about frequent citation is correct, but the observation does nothing to validate the opinion cited. Berger offered no comments or analysis in critique of the frequent miscitation of Ferebee, leaving the reader to believe that citing Ferebee for the sufficiency of treating physician opinion without data was somehow appropriate. The citations to which Berger referred were erroneous in 1994, and they remain erroneous today.

[1] Matrixx Iniatives, Inc. v. Siracusano, 563 U.S. 27, 131 S.Ct. 1309, 1320 (2011).

[2] Brief for the United States as Amicus Curiae, in Matrixx Iniatives, Inc. v. Siracusano, No. 09-1156, 2010 WL 4624148, at *15 (Nov. 2010).

[3] See, e.g., Benkwith v. Matrixx Initiatives, Inc., 467 F. Supp. 2d 1316, 1326, 1330, 1332 (M.D. Ala. 2006) (granting defendant’s motion to exclude testimony of an expert in the field of epidemiology regarding Zicam nasal spray’s causing plaintiff’s anosmia, because the opinions had not been tested and a rate of error could not be provided).

[4] Sheldon L. Wagner, CLINICAL TOXICOLOGY OF AGRICULTURAL CHEMICALS 198, 199-200 (1983).

[5] Id. at 200 (citing “[s]tudies by Staiff and co-workers (1975)” on occupationally exposed persons).

[6] Id. at 201. See also A. J. Gardiner, Pulmonary oedema in paraquat poisoning, 27 THORAX 132 (1972).

[7] WORLD HEALTH ORGANIZATION, ENVIRONMENTAL HEALTH CRITERIA 39: PARAQUAT AND DIQUAT at § 1.1.5. Effects on man (1984).

[8] See K. Tungsanga, S. Chusilp, S. Iarasena & V. Sitprija, Paraquat poisoning: evidence of systemic toxicity after dermal exposure, 59 POSTGRAD. MED. J. 338, 338 (1983).

[9] Cf. Zuchowicz v. United States, 140 F.3d 381 (2nd Cir. 1998) (analyzing causation in the context of defendants clear negligence that resulted in undisputed overexposure to prescription medication Danocrine). Unlike Zuchowicz, however, the Ferebee case did provide a strong evidentiary base for causation.

[10] See Kenneth J. Chesebro, Taking Daubert’s “Focus” Seriously: The Methodology/Conclusion Distinction, 15 CARDOZO L. REV. 1745, 1747, 1753 (1994) (misciting Ferebee as a Rule 702 case).

[11] See Alani Golanski, Judicial Scrutiny of Expert Testimony in Environmental Tort Litigation, 9 PACE ENVT’L L. REV. 399, 406-07 (1992) (misrepresenting Ferebee as a case under Rule 703; “this evidentiary issue was resolved through examination of the facts or data underlying the proffered expert opinion only to the extent necessary to make a Rule 703 determination on whether they are of the type reasonably relied upon by experts in the field.”). See also Michael C. McCarthy, “Helpful” or “Reasonably Reliable”? Analyzing the Expert Witness’s Methodology Under Federal Rules of Evidence 702 and 703, 77 CORNELL L. REV. 350, 373 (1992) (discussing Ferebee as a Rule 702 and 703 decision).

[12] Richardson v. Richardson-Merrell, Inc., 857 F.2d 823, 831-832 (D.C. Cir. 1988).

[13] Ferebee, 736 F.2d at 1536.

[14] Id.

[15] Louis Lasagna & Sheila R. Shulman, Bendectin and the Language of Causation, chap. 5, at 111, in Kenneth R. Foster, David E. Bernstein & Peter W. Huber, eds., PHANTOM RISK: SCIENTIFIC INFERENCE AND THE LAW (1993)

[16] See Michael C. McCarthy, “Helpful” or “Reasonably Reliable”? Analyzing the Expert Witness’s Methodology Under Federal Rules of Evidence 702 and 703, 77 CORNELL L. REV. 350, 373 (1992) (“Essentially, the Ferebee decision distinguished between the level of certainty required by a scientific discipline-and the level of certainty required by a court in drawing conclusions regarding causation”: and “Ferebee stands for the proposition that courts, in determining whether a given substance more likely than not caused a plaintiff’s injury, cannot always wait for the sciences.”).

[17] Ferebee, 736 F.2d 1529, 1535 (D.C. Cir. 1984).

[18] David E. Bernstein, The Misbegotten Judicial Resistance to the Daubert Revolution, 89 NOTRE DAME L. REV. 27, 36 (2013); David E. Bernstein, Expert Witnesses, Adversarial Bias, and the (Partial) Failure of the Daubert Revolution, 93 IOWA L. REV. 451, 465 (2008) (“Ferebee involved a claim that exposure to an herbicide caused an individuals’ cancer.”).

[19] Donaldson v. Central Illinois Public Service Co., 313 Ill. App.3d 1061, 730 N.E.2d 68, 79 (2000).

[20] Id.

[21] Lee Loevinger, Evidentiary Framework Margaret A. Berger Reference Manual on Scientific Evidence, 36 JURIMETRICS J. 149, 153 & n.21 (1996) (discussing the before (Daubert) times when “mere qualification and the facial relevance of an opinion might suffice to let an expert testify in some jurisdictions.”).

[22] See Kenneth J. Chesebro, Taking Daubert’s “Focus: Seriously: The Methodology/Conclusion Distinction, 15 CARDOZO L. REV. 1745, 1747 & n.20 (1994) (incorrectly arguing that Ferebee involved an expert witness offered an “unprecedented expert factual conclusion which no published literature supported.”

[23] See Margaret A. Berger, Evidentiary Framework, 39, 81 & n.164, in FEDERAL JUDICIAL CENTER, REFERENCE MANUAL ON SCIENTIFIC EVIDENCE (1st ed. 1994).

Posted in Causation, Expert Witnesses, Rule 702, Rule 703, Scientific Evidence, statistical evidence, Toxicological Evidence | 2 Comments »