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Expectant mothers who take Tylenol to relieve their pregnancy pains may be increasing their children's risk of attention-deficit/hyperactivity disorder, according to a new study.

While acetaminophen, the active ingredient in Tylenol, is the most commonly used drug for pain and fever during pregnancy, some recent studies reveal that the medication can negatively affect neurodevelopment and cause behavioral dysfunction in unborn children by altering sex and other hormones.

The latest study involved 64,322 children and mothers in the Danish National Birth Cohort between 1996 and 2002. Parents were asked to report behavioral problems on a questionnaire and hyperkinetic disorder diagnoses and medication prescriptions for attention-deficit/hyperactivity disorder were collected from Danish registries.

The study showed that more than half of mothers said they used acetaminophen while pregnant. Researchers found that children of mothers who used acetaminophen during pregnancy were more likely to be diagnosed with hyperkinetic disorders, have ADHD-like behaviors and use ADHD-medications by age seven.

The findings revealed that the risk of ADHD was more pronounced in children of mothers who used acetaminophen in more than one trimester during pregnancy.

"Maternal acetaminophen use during pregnancy is associated with a higher risk for HKDs and ADHD-like behaviors in children. Because the exposure and outcomes are frequent, these results are of public health relevance but further investigations are needed," researchers concluded.

In an accompanying article, Miriam Cooper of Cardiff University School of Medicine in Wales, who was not part of the study, cautioned that the findings do not prove causation.

"An interesting new study in this issue of the journal has found preliminary evidence that prenatal exposure to a drug considered safe in pregnancy (acetaminophen or paracetamol) may be associated with ADHD in childhood," Cooper wrote in the article.

"Indeed, causation cannot be inferred from the present observed associations, and Liew et al are right to point out that a replication of their study is needed," she added.

"In summary, findings from this study should be interpreted cautiously and should not change practice. However, they underline the importance of not taking a drug's safety during pregnancy for granted, and they provide a platform from which to conduct further related analyses exploring a potential relationship between acetaminophen use and altered neurodevelopment," Cooper concluded.

The findings are published in the journal JAMA Pediatrics.