LONDON - ANTI-IMPOTENCE drug Viagra can damage fertility and prevent some men from starting a family, scientists have warned.

The new research, which suggests that the drug can damage sperm, serves as a warning to young men who take the drug recreationally, the Telegraph reported on Sunday.

It also warned that fertility clinics prescribing the drug to help men produce sperm could be preventing couples from getting pregnant.

The team led by Dr David Glenn, a consultant gynaecologist at Queen's University Belfast, bathed sperm samples in weak solutions of Viagra and found that it made the sperm more active.But it also found that such sperm had a damaged acrosome - the part containing the enzymes needed to break down the membrane surrounding a woman's egg to allow sperm to fertilise it. Hence, it was unable to get into the egg to fertilise it.

'Essentially, the acrosome breaks open too early in sperm that has been exposed to Viagra. The sperm cannot get to the egg, and so it is not fertilised,' Dr Glenn said.

Similar experiments on mice showed that those administered Viagra produced 40 per cent fewer embryos.

Listing the main concerns raised by the findings, Dr Glenn said Viagra, which is widely mixed with cocaine and sold in clubs, 'could pose serious fertility problems later in life'.

The other concern was the potential harm caused by the practice of some fertility clinics of giving Viagra to men.

'Couples (going to fertility clinics) are already having problems getting pregnant. Giving male partners something that could make the problem worse is scarcely the right approach,' Dr Glenn said.

The study will be published in the journal Fertility And Sterility.