A new study published in JAMA Internal Medicine found no evidence that acetaminophen use during pregnancy causes autism or ADHD, undercutting claims from President Donald Trump and Health Secretary Robert F. Kennedy Jr. that have already pushed some expecting mothers away from Tylenol.
The researchers examined electronic health records in Hong Kong from 2001 through 2023, covering more than 700,000 mother-child pairs. About 43% of the children in the data set had prenatal exposure to acetaminophen, the active ingredient in Tylenol.
According to Ars Technica’s summary of the study, the researchers found no link between acetaminophen exposure in the womb and either autism or ADHD. That held across dosage, pregnancy trimester, frequency of use and the mother’s age. In other words, the usual places one would expect a dose-response signal to show up were blank.
That result lands in the middle of a very political mess. Trump has claimed autism must have an external environmental cause, while Kennedy said he would identify a root cause of autism. The Trump administration later told pregnant women to avoid Tylenol over an unproven autism claim, according to Techdirt’s prior reporting. Kennedy has since acknowledged that no proven causal link exists, Techdirt reported.
Why the correlation keeps appearing
The study did find an association when researchers used a cruder comparison: children exposed to acetaminophen before birth versus children who were not. That is the kind of result that keeps this claim shambling around public health debates like a badly labeled spreadsheet.
The problem is that the association disappeared under stronger tests. The researchers used a sibling-matched design, comparing siblings within the same family rather than treating every child as an unrelated data point. That method helps control for shared family traits, genetics and environment, the stuff that can quietly contaminate a simple exposed-versus-unexposed comparison.
The researchers also ran a negative control analysis. In plain English, that means testing a scenario where the supposed biological pathway should not work. They compared children whose mothers used acetaminophen before pregnancy or after birth with children whose mothers did not use the drug. That comparison also produced an association, Ars Technica reported, even though it would be biologically implausible for acetaminophen taken outside pregnancy to cause prenatal neurodevelopmental effects.
That is a bright red warning label on the simple correlation. If acetaminophen use before conception or after delivery tracks with autism diagnoses too, the drug exposure during pregnancy is probably standing in for something else: family factors, genetic factors, environmental conditions, or health issues that make acetaminophen use more likely and are also associated with autism or ADHD diagnoses.
Politics got ahead of the evidence
Kenvue, the maker of Tylenol, has also faced lawsuits over the alleged autism link. Techdirt pointed specifically to a suit by Texas Attorney General Ken Paxton. The new JAMA Internal Medicine study does not decide those legal claims, but it adds another large data set to the scientific record against a causal theory that has not been proven.
The difference between correlation and causation is not academic pedantry here. Public officials told pregnant women to avoid a common painkiller on the basis of an unproven claim. The latest large study says the causal signal still is not there.
This story draws on original reporting from Techdirt.
