Emma Simpson on the medical phenomenon sweeping America

Fifty-year-old Earl Thompson is a quiet-spoken man. For months he wouldn't seek help for his impotence, but was eventually persuaded to take part in a trial for Viagra.

Earl Thompson: "It's a miracle."

For him, the results were astonishing. "It's a miracle drug," he says. "Life doesn't end at fifty, it goes on."

Earl's wife Gwen is even more forthright in her praise for the new treatment: "We just felt rejuvenated, our whole relationship became reborn because of this drug. I cannot say enough positive things about it."

An oral revolution

Very little is known about the prevalence of impotence - or erectile dysfunction as it is known - in the United States. Recent estimates suggest that the number of US men with the condition may be near 10-20 million. Not surprisingly, it is difficult to get reliable figures because many men are reluctant to talk it.

Dr Jacob Rajfer: "revolutionary"

Existing treatments - using pumps and injections - lack the spontaneity demanded in the bedroom. This helps to explain why there has been such a stampede for viagra: it's just a simple pill.

Pharmacist Homer Namjo, like so may in his profession, is having difficulty meeting the demand. "It's been amazing," he says. "I've never seen anything like it. I was getting requests before it came on to the market. I get five or six people a day looking for it."

Dr Jacob Rajfer at the UCLA School of Medicine has signed 400 prescriptions alone. He thinks the drug can help two out of every three patients.

"I believe the drug is extremely safe and extremely effective," he says.

"Being an oral pill for the treatment of impotence, it is certainly revolutionary, and for many patients it has been life changing."

The ups and downs

But Viagra does carry some risk. Already, six deaths have been reported in the US. Cardiologist Dr Michele Hamilton sees many men with a heart condition who also have an impotence problem. She warns that some patients may be in grave danger if they take viagra.

Low blood pressure and viagra do not mix

"Viagra works by dilating blood vessels. Any medicine that dilates blood vessels can lower blood pressure. So any patient who has low blood pressure, because their heart does not work well or is on blood pressure medicines, has to be careful because it could possibly lower their blood pressure further and make them pass out."

Viagra's manufacturers Pfizer insist the drug is safe, and users should always seek medical advice before taking it. Dr Jacob Rajfer also warns against the casual use of the drug: "The drug is not an aphrodisiac, and if your are normal in your sexual function I don't think you will see much of a difference with this medicine."