Scientists at the University of Edinburgh 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 follow existing guidelines that the painkiller be taken at the lowest effective dose for the shortest possible time.
Research carried out on mice found that rodents given three doses of paracetamol a day for a week had a 45 per cent reduction in testosterone compared to 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, testicular cancer and undescended testicles.
The study saw scientists test the effect of paracetamol on mice carrying grafts of human testicular tissue, which have been shown to mimic how the developing testes grow and function during pregnancy.
They gave the mice a typical three-times daily dose of paracetamol over a period of 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 per cent.
Current NHS guidance on paracetamol use during pregnancy advises that, as with any medicine, it should be taken at the lowest effective dose for the shortest possible time.
Dr Rod Mitchell, of the University of Edinburgh, said: “This study adds to existing evidence that prolonged use of paracetamol in pregnancy may increase the risk of reproductive disorders in male babies.
“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.”
The Royal College of Midwives’ head of education, Carmel Lloyd, said: “If women do take medicines such as paracetamol when they are pregnant, they should use the lowest effective dose for the shortest possible time. If the recommended dose doesn’t control their symptoms or they are often in pain, they should seek more advice from their midwife or doctor.
“Ideally, women should avoid taking medicines when they are pregnant, particularly during the first three months. Minor conditions such as colds or minor aches and pains often do not need treating with medicines. If women feel they need to take medicines such as paracetamol when they are pregnant, they should talk to their midwife or doctor first; they can also get advice from their local pharmacy.”
Dr Martin Ward-Platt, of the Royal College of Paediatrics and Child Health, said: “The findings of this study send a clear message – expectant mothers should not prolong paracetamol use during pregnancy, only taking it when necessary and as per current NICE guidelines.
“However, the study specifically relates to paracetamol use over at least several days. There are times where one or two doses is needed to treat one-off episodes of fever for example. Fever during pregnancy can be harmful to the developing embryo, with links to a significant increase in the rates of spina bifida and heart malformations, so small doses of paracetamol are sometimes necessary.
“My message to expectant mothers is clear – avoid over-use of paracetamol but if you do have a fever, or any other sort of pain where you would normally use paracetamol, seek medical advice.”
The research is published in the journal Science Translational Medicine.