Taking painkillers during pregnancy could affect the fertility of the unborn child in later life, new research suggests.
Edinburgh University find the drugs may also affect the fertility of future generations, by leaving marks on DNA.
Experts said the findings added to growing evidence some medications, including Paracetamol, should be used with caution during pregnancy.
Researchers emphasized advice for pregnant women remained unchanged.
Current guidelines tell, if necessary, Paracetamol should be used at the lowest possible dosage for the shortest possible time. Ibuprofen should be avoided during pregnancy.
The study looked at the effects of Paracetamol and ibuprofen on samples of human foetal testes and ovaries.
The scientists find similar effects using several different experimental approaches, including laboratory tests on human tissue samples and animal studies.
Human tissue exposed to either narcotic for one week in a dish had reduced number of germ cells that give rise to sperm and eggs, cells, the study found.
Ovaries exposed to Paracetamol for one week had more than 40% fewer egg-producing cells. After ibuprofen exposure, the number of cells was almost halved.
Experts said this was important because daughters make all of their eggs in the womb, so if they are born with a reduced number it could lead to an early menopause.
Painkiller exposure during developing could have effects on unborn boys too, such studies found.
Testicular tissue exposed to analgesics in a culture dish had around a quarter fewer sperm-producing cells after exposure to Paracetamol or ibuprofen.
The team also tested the effects of painkiller treatment on mice that carried graftings of human foetal testicular tissue.
The graftings have been shown to imitation how the testes grow and function during developed as the womb.
After only one day of treatment with a human-equivalent dose of Paracetamol, the number of sperm-producing cells in the grafting tissue had dropped by 17%. After a week of drug therapy, there were almost one third fewer cells.
Previous studies with rats have shown that painkillers administered in pregnancy led to a reduction in germ cells in female offspring. This affected their fertility and the fertility of females in subsequent generations.
The scientists found that exposure to Paracetamol or ibuprofen triggered mechanisms in the cell that constructed changes in the structure of DNA, called epigenetic marks.
The marks can be inherited, helping to explain how the effects of painkillers on fertility may be passed on to future generations.
Painkillers’ effects on germ cells are likely caused by their actions on molecules called prostaglandins, which have key functions in the ovaries and testes, the researchers found.
The study, published in Environmental Health Perspectives, was funded by the Medical Research Council, Wellcome and the British Society of Paediatric Endocrinology and Diabetes.
Dr Rod Mitchell, who led the research at the University of Edinburgh’s MRC Centre for Reproductive Health, told: “We would encourage women to think carefully before taking painkillers in pregnancy and to follow existing guidelines – taking the lowest possible dose for the shortest possible time possible.”