Prolonged paracetamol use by pregnant women may reduce testosterone production in their unborn sons.
Scientists at Edinburgh University said their study adds to existing evidence that too much paracetamol in pregnancy may increase the risk of reproductive disorders in male babies.
They said expectant mothers should take the painkiller at the lowest effective dose for the shortest possible time.
Research carried out on mice found that those given three doses of paracetamol a day for a week had a 45pc reduction in testosterone compared with a placebo.
The hormone, which is produced in the testicles, is crucial for life-long male health. Reduced exposure to testosterone in the womb has been linked to an increased risk of infertility and testicular cancer.
Scientists tested the effect of paracetamol on mice carrying grafts of human testicular tissue. They gave the mice a three-times daily dose of paracetamol over either 24 hours or seven days, and measured the amount of testosterone produced by the human tissue an hour after the final dose of paracetamol.
They found there was no effect on testosterone production following 24 hours of paracetamol treatment, but after a week of exposure the amount of testosterone was reduced by 45.
"This study adds to existing evidence that prolonged use of paracetamol in pregnancy may increase the risk of reproductive disorders in male babies," said Dr Rod Mitchell of Edinburgh University .