I first heard about taking Viagra for fun when my boyfriend's housemate Dominic bounded into the kitchen one morning, grinning and flushed with a new-found cockiness.

'What's the greatest number of times you guys have had sex in one night?' he demanded. 'Take that number and double it, because that's a night on Viagra . You've got to try it. You both should.'

I laughed, but wasn't too surprised - there are few things my friends wouldn't try. We have good jobs but few responsibilities. Even though some of us are doctors, journalists, management consultants and lawyers, we spend our free time indulging ourselves like wealthy, overgrown teenagers.

It turns out that many of my friends have enjoyed taking Viagra , either on its own or in combination with other recreational drugs. If you are used to taking illegal substances at the weekend, the prospect of popping something medically approved shouldn't be very daunting.

My friends have a few cautionary tales, though. One says Viagra stops him from sleeping properly. Another complains of a blue haze affecting his vision whenever he takes it. And my friend Rahul once found it an embarrassing inconvenience. 'I popped it one time and failed to get laid,' he tells me. 'I walked back from the girl's house the next day, and the mere chaffing of my boxer shots against my penis as I walked turned me on.'

If I am being offered Viagra , then there must be a large market for it on the recreational scene. I decide to investigate.

Viagra was initially designed as a heart drug, but clinical trials showed it was no more effective than cheaper medicines already on the market. When Pfizer tried to abandon the trial, the male patients refused to stop taking it. It was only then that the side effect that has made famous was discovered.

Dr Graham Jackson, a consultant cardiologist at Guy's hospital, was part of the original research team when Pfizer developed Viagra . He explains that there are actually three different drugs on the market: Viagra , Cialis and Levitra. Levitra and Viagra work in about an hour and last around six hours, but Cialis takes two hours to work and lasts up to 48 hours.

'There's a recreational circus going on with these drugs,' he says. 'You can get Viagra from any cab driver now. Nothing much would happen if someone without erectile dysfunction took Viagra, although they might think they feel better, psychologically. But it would make a difference if you took it along with other drugs. Cocaine, marijuana, ecstasy, alcohol, nicotine, crystal meth - they all can cause erection problems. Viagra dilates the blood vessels and allows blood to flow into the penis when these drugs are stopping it.'

Little is known about the effects of Viagra on women, and Dr Jackson thinks results will never be proven clinically. Pfizer set up a female sexual dysfunction unit a few years ago, comparing Viagra in women to a placebo, but the study had to be stopped because 85 per cent of the women responded to the placebo. It seems that even the very idea of Viagra is enough to make some women aroused.

'There's no documented evidence of any major catastrophe with people taking these drugs recreationally, but it's not the sort of thing that's reported,' he says. I'm surprised when he tells me Viagra could even make certain illegal drugs safer: 'Cocaine is a very dangerous drug because it causes coronary arteries to shut down and can give you a massive heart attack. It's dreadful. And in a funny way, Viagra can actually help a cocaine user, because it does the opposite thing to the blood vessels.'