A new study is the first to show a connection in male mice exposed in utero to acetaminophen (the active ingredient in Tylenol) and reduced masculinization of the adult brain. The exposed mice were less aggressive and less prone to mate with females than mice not exposed. Exposure to acetaminophen in the mice was at levels comparable to maximum doses regarded as safe for pregnant women.

Earlier studies by these authors show that prenatal exposure to acetaminophen may also result in developmental changes in the reproductive organs in rodents and humans, resulting in, for example, female infertility among adult rodents and malformations in newborn boys. These findings are of particular concern as acetaminophen is routinely recommended by physicians as safe for use while pregnant.

The authors say their findings should be seen in relation to other recent, peer-reviewed findings. These studies show that changes in genital development in humans and rodents likely occur through inhibition of sex hormones associated with fetal exposure to acetaminophen. This inhibition of sex hormones like testosterone is shown in this study not only to have consequences for the genitals, but also for programming of the male brain. It is well known that testosterone is needed for proper male development of the brain, and acetaminophen has been shown to block this hormone.

“Prenatal exposures to paracetamol/acetaminophen and precursor aniline impair masculinisation of male brain and behaviour” will be published in the peer-reviewed journal Reproduction on June 22. A tip sheet on prenatal exposure to acetaminophen and associated adverse health effects in humans and rodents is also available.

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