When we have a headache or a cold, many of us pop a Tylenol without a second thought. But acetaminophen - the active analgesic ingredient in the drug - is also commonly used to ease pain during pregnancy. A new study suggests that this could be a major problem.

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New research warns about the adverse effects that prenatal exposure to acetaminophen may have on male children.

The new research suggests that taking the common analgesic acetaminophen during pregnancy is not a good idea, especially for mothers expecting male babies.

The study, published in the journal Reproduction, examines the effects of acetaminophen in mouse fetuses and finds adverse effects on the masculinization of the mouse brain, extending all the way into adulthood.

Previous research has already indicated that acetaminophen can suppress the development of testosterone in male fetuses, potentially leading to developmental changes in the reproductive system and the brain.

Testosterone is a male sex hormone that drives the growth and development of the male body, as well as the "male programming" of the brain. In men, testosterone controls sex drive, bone and muscle mass, fat distribution, and the production of sperm and red blood cells.

In the previous studies in rodents, inhibited levels of testosterone in the fetuses were shown to raise the risk of testicular malformation in newborns.

But there are other health risks posed by inhibited levels of testosterone, and many of them manifest in the behavior of adult males, suggests the new research.

The first author of the study is Prof. Anders Hay-Schmidt, who, at the time of the study, was part of the Department of Neuroscience and Pharmacology at the University of Copenhagen in Denmark.

Acetaminophen inhibits masculinization

Prof. Hay-Schmidt and colleagues gave mice a dose of acetaminophen almost equivalent to that which pregnant women are usually recommended.

The researchers evaluated the male rodents' behavior, looking at their aggressiveness toward other males and their ability to mark their territory, as well as their ability to mate.

As adults, the mice whose mothers had received acetaminophen performed significantly worse across all of the three criteria.

Prenatal exposure to acetaminophen resulted in alterations in the adults' urinary marking behavior. The rodents were also less aggressive toward males invading their territory. The mice also had "reduced intromissions and ejaculations" during mating.

The behavioral changes noticed by the researchers were also backed up by investigations into the mice's brains. The researchers found that the number of neurons had significantly decreased in the brain region that controls sex drive.

Specifically, in the acetaminophen-exposed male mice, the brain area called the "sexually dimorphic nucleus" in the anterior hypothalamus had "half as many neurons as the control mice."

The corresponding author of the study, Dr. David Møbjerg Kristensen - of the Department of Biomedical Sciences and the Novo Nordisk Foundation Center for Protein Research at the Faculty of Health and Medical Sciences - comments on the findings.

He says, "We have demonstrated that a reduced level of testosterone means that male characteristics do not develop as they should. This also affects sex drive."

"In a trial, mice exposed to paracetamol at the fetal stage were simply unable to copulate in the same way as our control animals. Male programming had not been properly established during their fetal development, and this could be seen long afterwards in their adult life. It is very worrying."

Dr. David Møbjerg Kristensen

He recommends taking the painkiller with caution and also reminds future mothers to consult their physician if they are unsure about the medication they wish to take.

"I personally think that people should think carefully before taking medicine. These days it has become so common to take paracetamol that we forget it is a medicine, and all medicine has side effects. If you are ill, you should naturally take the medicine you need. After all, having a sick mother is more harmful for the fetus," says Dr. Kristensen.

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