Doctors worn that the increase may lead to health problems, including sexually related diseases, unknown interactions with other drugs, and possible effects on the heart.

"We were taken aback by the increase. We had not realised it was so widely prevalent, especially among women, " said Dr Jim McCambridge, research fellow at the National Addiction Centre, Institute of Psychiatry.

He and his colleagues, who report their findings in the journal Drug and Alcohol Review, this week called for better monitoring of the use and effects of the drug sildenafil, or Viagra, which was first licensed as a treatment for men in 1998.

Although it's known that the prescription-only drug can be bought over the internet, the extent of its illicit use is not really known. The new research, based on questionnaires completed by more than 4,000 clubbers across Britain over a period of five years, shows that increasing numbers of men and women are using Viagra, either on its own or with other drugs.

The results show that the number of clubbers using the drug is climbing by around 7% every year. In the first year of the study, 3.2% of men and 1.1% of women used it, but by the fifth year of the study, the proportions had climbed to 17% of men and 13% of women. Exactly what effect it has, or what it is used for, is not clear, although one theory is that it is used to increase arousal and enhance the effects of the other drugs.

"What is worrying is that the potential effects and health hazards are unknown for women because it is not used for women.

There is very little research in this area. This drug is not licensed in the UK for women, so all female use would appear to be on an unprescribed basis, carrying with it unknown consequences, " said McCambridge.

The woman: It was on a night out last New Year's Eve that Jo Smith, 26, a customer services representative from Staines, "rst tried Viagra.

"I had gone out with a load of friends to the pub and one of them had some so we tried it just for a laugh."

Her first time turned out not to be her last and she has had some good times on the drug since.

"There was the time I went up to Liverpool for a massive night out. It was after the races at Aintree last summer and I took some. I have to say that come the evening I did feel horny and it did increase my awareness.

"Really, I think it is more psychological than anything else. It is part of the recreational drug culture that is out there. I think that more women are taking it now, particularly those in the city . . . but I think it is more of a social status thing, trying to be part of the 'in crowd', than anything else. I don't really see it as a dangerous drug.

When you think about it, there is always an element of risk with everything that you do in life."

The man: Nicky Hutchinson, 30, works in creative media in London and is a seasoned clubber, having gone out most weeks for over 10 years. He sees Viagra as a pick-me-up at the end of a night out.

"I remember that the person I was with at the time suggested we try it. Taking Viagra with other drugs meant what might have lasted an hour went on for five hours. I have to say it really does work, but it does hit you hard the next day."

He is now an occasional user. "Viagra is not addictive and you can still perform naturally without it, which was a worry to start with."

Clubbers who do certain drugs can often have problems getting an erection, according to Nicky, but he says that Viagra cancels this out and is widely used.

"Most people I know use it now. People are taking it because they can get high and still have sex for hours afterwards. You can buy it anywhere and get it delivered to you in a couple of hours. Viagra is much the same sort of price as you'd pay for an E, with a pill in a club generally costing a fiver these days."