Women who take paracetamol during pregnancy may have sons with less masculine traits – and a lower sex drive.
The painkiller, often prescribed to pregnant women for pain and fever, can alter the male brain.
It is thought to block testosterone when babies are developing in the womb.
As a result, boys could show less typically masculine characteristics and be sexually dysfunctional, a study published in the journal Reproduction suggests.
These results have so far been proven only in mice, due to concerns about giving women paracetamol in pregnancy. Experts stress that pregnant women typically take painkillers for much shorter periods than the daily dose given to mice, but co-author Dr David Mobjerg Kristensen still called the findings "worrying".
He said: "These days it has become so common to take paracetamol that we forget it is a medicine. And all medicine has side-effects." Paracetamol is the world’s most popular painkiller and the only one deemed suitable to take during pregnancy. Last year University of East Anglia researchers said it was "perfectly safe".
But the latest study, led by the University of Copenhagen, found that the painkiller curtails development of "male behaviours". The authors say it blocks male hormones in the brain vital to whether the brain of an unborn baby becomes masculine or feminine.
When given to mice in similar doses approved for pregnant women, their male offspring were less able to mate with females, less aggressive, and displayed territorial behaviour seen usually in females. Previous research has shown similar drugs called phthalates cause young boys to spend less time playing traditionally masculine games.
Studies have also shown paracetamol’s properties increase the risk of malformed testicles.
Dr Kristensen said: "We have demonstrated that a reduced level of testosterone means that male characteristics do not develop as they should. It is very worrying." It follows a study last year by the same group showing female mice became infertile at a younger age if their mothers had paracetamol during pregnancy.
However this does not mean that paracetamol must never be taken during pregnancy. Dr Kristensen said: "If you are ill, you should naturally take the medicine you need. After all, having a sick mother is more harmful for the foetus."
Dr Rod Mitchell, research group leader of the MRC Centre for Reproductive Health at the University of Edinburgh, said the daily doses of paracetamol given to mice over several weeks of pregnancy did not reflect how pregnant women typically take the drug – for a 24- or 48-hour period.
He said: "The findings ... raise the possibility that prolonged exposure to paracetamol might affect masculinisation of the brain."
But he stressed: "It is important to recognise that management of pain and fever during pregnancy are important for the health of mother and baby."