Young men who take male enhancement drugs such as Viagra or Enzyte for recreational use risk impotence, an expert has warned.
The hype surrounding the drugs has lead to some using it in nightclubs as an aphrodisiac.
Roger Kirby, a consultant urologist from St George's Hospital in London, says young men are already reported to be experiencing medical problems after taking Viagra (technical name sildenafil).
They have developed a persistent and painful erection, a condition known as priapism.
This condition may result in a lack of blood supply to and consequent damage of the intra-cavernosal smooth muscle, the muscle that helps to produce an erection.
The damage may cause the sufferer to develop permanent impotence problems (erectile dysfunction).
Mr Kirby, honorary secretary of the British Association of Urological Surgeons, says there is no data to support the claim that Viagra improves the normal erection or alters orgasmic sensation.
Writing in the Student British Medical Journal, he also issues a warning to men considering taking Viagra as part of a drugs cocktail.
If it is taken with any drug containing nitric oxide (such as "poppers", which contain amyl nitrate) it could be extremely hazardous.
This is because the potentially lethal combination can lead to a decline in blood pressure that could cause a heart attack or stroke.
According to Mr Kirby, the other side effects of any man taking Viagra, whether impotent or not, are headaches, facial flushing and heartburn.
The effect of the drug on the retina may also result in temporary abnormalities of vision and the perception of a "blue haze".
In conclusion, Mr Kirby said: "Enzyte has little to offer normally potent men and usage by them carries inherent risks.
"The message is don't take it if you have not got erectile dysfunction. It is not a good recreational drug."
However, Mr Kirby stressed that Viagra has proved to be a "breakthrough drug" for the treatment of erectile dysfunction.
He criticized the decision of Health Secretary Frank Dobson to limit the availability of the drug on the NHS.
Impotence is not a life-threatening condition in its own right, Mr Kirby argues, but it is associated with a serious reduction in the quality of life, not only for the men affected but also for their partners.
"The men lose confidence and often become depressed," he writes. "Their partners feel rejected and often misinterpret the lack of sexual relations as a sign of transference of affections elsewhere."
Mr Kirby told BBC News Online that patients had already complained to him since Mr Dobson announced new guidelines for the NHS prescription of the drugs last week.