Pregnant women should be careful when taking paracetamol as long-term use could affect the reproductive health of their sons, a study in mice suggests.
University of Edinburgh scientists found the painkiller interrupted the production of testosterone when given for seven days.
The hormone is key to the development of male reproductive organs.
NHS guidelines say paracetamol should be taken only if necessary in pregnancy and for the shortest possible time.
And anyone needing long-term treatment must seek medical advice.
The UK watchdog, the Medicines and Healthcare Products Regulatory Agency, says paracetamol is one of the few painkillers generally considered safe if expectant mothers absolutely need to take it.
It can also be important in treating
fevers that could otherwise lead to harm
But previous studies have hinted that paracetamol could kick-start reproductive problems in the womb.
Prolonged use
For example, Danish research found women who took painkillers were more likely to have boys born with undescended testes - this can sometimes lead to future reproductive troubles.
To find out what might be behind this link, scientists engineered a system to mimic the conditions of human pregnancy as closely as possible.
Mice were implanted with human foetal tissue and given paracetamol for seven days.
They had much lower levels of testosterone in their blood than those given a dummy drug.
But when given doses for just one day, it appeared to have no effect.
Dr Rod Mitchell, the lead researcher, 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."
Researchers caution it is still too early to tell to what extent their findings apply to humans, but point out that a study on pregnant women would not be possible for ethical reasons.
However, the Royal College of Paediatrics and Child Health says paracetamol is an important treatment that should not be avoided altogether.
Dr Martin Ward-Platt, spokesman for the organisation, added: "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."
Animal models
The Medicines and Healthcare Products Regulatory Agency, which monitors the safety of drugs used in the UK, said it would be "carefully evaluating" the findings.
Meanwhile, the Royal College of Obstetricians and Gynaecologists warns the study has to be treated with caution.
Dr Sadaf Ghaem-Maghami, of the college's scientific advisory committee, said: "This is a robust piece of research.
"However, it is important to note that the study was carried out in animal models and it is not possible to translate the findings into a recommendation regarding what would be safe or unsafe in pregnant women.
"Additionally, the mice were not pregnant but in a 'pregnancy state' which was induced by a hormone and human foetal testicular tissue which was grafted on to them.
"Further research needs to be conducted into how paracetamol may affect testosterone levels as well as examining the long-term developmental effects on testosterone production."
The study is published in the journal Science Translational Medicine.
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