Pregnant women who take the painkiller paracetamol regularly for long periods may put their unborn sons' testosterone levels at risk, leading to possible reproductive problems later in life, researchers said on Wednesday.

In a study using mice with grafts of human tissue, the scientists found that a week's paracetamol treatment led to a sharp fall in the production of testosterone, a hormone that is critical to men's life-long health.

"We would advise that pregnant women should follow current guidance that the painkiller be taken at the lowest effective dose for the shortest possible time," said Rod Mitchell, a clinical research fellow at Edinburgh University who led the work.

Paracetamol, known as Tylenol in the United States, is one of the most common medicines used to ease pain and reduce fever and is used routinely during all stages of pregnancy.

The study, published in the journal Science Translational Medicine, tested the effect of paracetamol on testosterone production in mice with grafts of human testicular tissue designed to mimic how testes develop and function in pregnancy.

Mitchell's team gave the mice a typical daily dose of paracetamol over a period of either 24 hours or seven days and then measured the amount of testosterone produced by the human tissue an hour after the final dose of paracetamol.

After 24 hours of paracetamol treatment, they found no effect on testosterone production, but after seven days of exposure, the amount of testosterone fell by 45 percent.

Mitchell noted that reduced exposure to testosterone in the womb has been linked to higher risk of infertility, testicular cancer and undescended testicles. He said these latest results add to evidence that prolonged paracetamol use in pregnancy "may increase the risk of reproductive disorders in male babies".

Sadaf Ghaem-Maghami, chair of the Royal College of Obstetricians and Gynaecologists' scientific advisory committee said the research was "robust" and had produced important findings, but that since it was in animals with human tissues, it was difficult to extrapolate its meaning for humans.

"Further research needs to be conducted into how paracetamol may affect testosterone levels," she said in a statement, adding that pregnant women should continue to follow guidelines and take the lowest effective paracetamol dose for the shortest possible time when necessary.