President Trump’s claim that pregnant women can give babies autism by using Tylenol is partly based on a study by a scientist who was previously paid to give expert evidence in a claim against the drug’s manufacturers.

Last month Dr Andrea Baccarelli, a senior public health academic at Harvard University, co-authored a review that linked acetaminophen — the active ingredient used in Tylenol — to autism and warned that pregnant women should “limit” their consumption of the drug.

During a press conference on Monday, Dr Marty Makary, commissioner of the Food and Drug Administration, cited Baccarelli’s study — along with two others — as justification for officially advising pregnant women against taking Tylenol. Dr Makary said Baccarelli’s review “found the overwhelming body of evidence points to an association” and quoted the Harvard academic directly as saying: “We cannot wait any longer.”

Robert F Kennedy Jr, the health secretary, President Trump, and Mehmet Oz, the administrator for Medicare and Medicaid, at the announcement of the supposed risks of Tylenol

SAUL LOEB/AFP/GETTY IMAGES

But in the summer of 2023 Baccarelli served as an expert witness during a class action lawsuit against Kenvue, the maker of Tylenol. A group of parents filed the suit in 2022 with a New York federal court, alleging their children’s autism and ADHD was a result of prenatal use of acetaminophen, which is called paracetamol outside North America.

The complainants engaged five experts, including Baccarelli, to testify in their case. But in December 2023 their testimonies were dismissed by a federal judge, Denise Cote, who found their views to be “unreliable”.

Of Baccarelli’s testimony, Cote said: “The discussion in his reports is incomplete, unbalanced and at times misleading. In general, Dr Baccarelli downplays those studies that undercut his causation thesis and emphasises those that align with his thesis.”

Baccarelli is the dean of Harvard’s TH Chan School of Public Health and specialises in how environmental exposures affect human health. Harvard confirmed he was a paid expert witness in the 2023 hearing, but would not say how much he was paid to testify.

Clippings from medical journals selected by the White House that report a link between Tylenol use during pregnancy and the development of autism and ADHD in childhood

SAUL LOEB/AFP/GETTY IMAGES

Judge Cote said all five of the experts who testified in the unsuccessful lawsuit against Kenvue “have not served to enlighten but to obfuscate the weakness of the evidence on which they purport to rely and the contradictions in the research.”

Baccarelli’s finding that there is an association between acetaminophen and autism has also been criticised by members of the scientific community.

Professor David Mandell, associate director for the Centre for Autism Research at the Children’s Hospital of Philadelphia, said Baccarelli’s methodology did not provide a “rigorous review of evidence”. He added that one of the studies examined by Baccarelli’s paper explored the autism-acetaminophen link without providing any measure of autism symptoms. “The statements in the discussion suggesting that the link is strong, rather than somewhat equivocal, combined with the unorthodox methods, raise serious concerns about bias,” he told The Times.

Baccarelli did not respond to a request for comment. A Harvard spokesperson said that Baccarelli had disclosed his role as an expert witness against Tylenol in his paper, which the Times has confirmed.

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In a statement shared by the White House on Monday, Baccarelli said he had discussed his scientific findings and the recommendation not to take Tylenol during pregnancy with health secretary Robert F Kennedy Jr and the National Institutes of Health, adding he appreciated “their interest in this study”.

Some studies have shown a link between pregnant women taking Tylenol and autism, but scientists say these findings are inconsistent and inconclusive.

Baccarelli is a public health academic and dean at Harvard University, did not respond to a request for comment from The Times

KENT DAYTON/HARVARD TH CHAN SCHOOL OF PUBLIC HEALTH”

For example, a study in 2024 funded by the US government investigated about 2.5 million Swedish children and found some evidence that acetaminophen use during pregnancy causes autism, ADHD or other intellectual disabilities — but this association disappeared when the scientists compared one sibling who had been exposed with another who had not. The study, which is the largest of its kind, concluded that environmental and other health factors, rather than the drug itself, may be responsible.

Autistic Self Advocacy Network, a US non-profit organisation, told the Times that Baccarelli’s review had failed to cite the Swedish study and that some of the papers included in his research “had biased weighting systems that include parental recall of Tylenol use over hard data”.

Members of the Coalition for Autism Scientists, a group of more than 250 American researchers, added that studies assessing whether acetaminophen causes autism are “difficult to conduct”.

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“It is highly irresponsible and potentially dangerous to claim links between potential exposures and autism when the science is far more nuanced and uncertain,” the coalition said earlier this month.

In response to Monday’s press conference, the coalition said the government’s announcement “alarms us researchers who committed our entire careers to better understanding autism. The data cited do not support the claim that Tylenol causes autism … and only stoke fear and falsely suggest hope when there is no simple answer.”

Autism diagnoses in the US have increased sharply since 2000, according to the US Centers for Disease Control and Prevention. Scientists partly attributed this rise to an increased awareness of autism and an expanding definition of the disorder. Researchers have also been investigating environmental factors.