TAKING too much paracetamol in pregnancy can affect testosterone production in unborn sons, according to new research.

Conditions which can affect men in later life, including infertility and testicular cancer, have been linked to reduced exposure to testosterone in the womb.

Now scientists at Edinburgh University have found that prolonged use of paracetamol in pregnancy could lower levels of the male hormone by as much as 45 per cent.

The researchers underlined the importance of expectant mothers following guidance on the use of the drug.

Paracetamol is the primary medicine used for managing pain and fever during pregnancy, but the official advice states that the painkiller should be taken at the lowest effective dose for the shortest possible time.

The Edinburgh University study tested the effect of paracetamol on testosterone production in mice carrying grafts of human testicular tissue - a set-up shown to mimic how the developing testes grow and function during pregnancy.

The mice received doses of paracetamol for different lengths of time, some for 24 hours and others over the course of a week.

The researchers found there was no effect on testosterone production following 24 hours of paracetamol treatment but after seven days of exposure, the amount of testosterone was reduced by 45 per cent.

Dr Rod Mitchell, clinical research fellow at Edinburgh University, said there was already evidence that prolonged use of paracetamol may increase the risk of reproductive disorders in baby boys.

However, this study - detailed in the journal Science Translational Medicine - is the first to look directly at the impact of paracetamol on testosterone production in the womb.

While more research is needed to understand why the drug might have such an effect, Dr Mitchell said: "Not everyone is aware that the guidance about use of paracetamol during pregnancy is out there and given the general attitudes of the general population towards use of paracetamol - which is that it is not always associated with side-effects - even just reminding people that there are recommendations about its use, is important."

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 ... 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."

Carmel Lloyd, head of education for The Royal College of Midwives, said: "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."