Viagra, the first oral drug able to cure male impotence, has been so hyped it has been credited with everything from saving marriages to starting a new sexual revolution. Although released less than two months ago in the United States, sales have outstripped even manufacturer Pfizer's wildest projections. Pharmacies have been besieged by men, and even some women, determined to try it.

Along with the exuberant responses, an extensive underground seems to be building up, with reports of the drug being smuggled into countries including Korea and Taiwan.

But amidst all the hype and ever-inflated claims of sexual prowess, there are some worrying signs. Last weekend, Pfizer reported there had been six deaths among men using Viagra. Although none was directly caused by the drug itself, such a high number of fatalities less than two months after its release is causing concern.

Added to that are fears among therapists that Viagra will prove a disappointment to some men, and may even exacerbate marital problems. The Viagra hype is so huge reports of its effect tend to follow the Emperor's New Clothes approach. Men who have taken it all say it is great, because no one wants to admit to poor sexual performance.

But one of Hong Kong's leading sexual therapists, Dr Ng Man-lun, a professor of psychiatry at the University of Hong Kong, said it was important men realised that Viagra was not an aphrodisiac and would not increase sexual desire. Nor could it automatically increase sexual pleasure.

'This drug is not an aphrodisiac. It will not increase sexual appetite or arousal. It will just increase the physiological response,' Dr Ng said.

The physiological sexual response in both men and women involves an increase in blood flow. In men it causes tissue in the penis to swell and harden, leading to an erection. Alcohol, ageing, nerve damage, stress and anxiety can all interfere with this blood flow, leading to an incomplete or failed erection, a condition known as impotence. Men who have experienced this, especially those who are very concerned about their sexual performance, can develop such anxiety about it they become permanently impotent.

Dr Ng said that for these men, Viagra does work like a miracle drug. 'It will establish erection no matter what a person is thinking about. So it can help in psychogenic impotence.' Viagra will also help doctors work out whether physical or mental problems are causing a man's impotence. Those who have severe blood-vessel damage caused by conditions such as diabetes will not get much of a response from Viagra, because the drug cannot get blood flowing through blocked blood vessels. But men with impotence caused by nerve damage will get a good response.

Men are flocking to obtain Viagra, which is not yet officially available on prescription in Hong Kong. But Dr Ng argues that it does not actually do anything older drugs now available in Hong Kong cannot do. It just does it in a more convenient way.

The drugs currently being used here have to be injected into the penis, and will also achieve 'nearly 100 per cent effect if applied to the right person', said Dr Ng. However, he added, not many men are keen on that approach.

While Viagra is very effective at inducing erection, all the claims about its wondrous effect on sexual relationships are likely to be untrue, said Dr Ng. 'It cannot help with every sexual problem. It may not really help sexual pleasure.' Most of Dr Ng's female patients suffer from lack of interest in sex, rather than frustration at having a partner unable to achieve erection. Dr Ng said the commonest sexual problem among Hong Kong women was lack of pleasure in the process and a wish to stop having sex altogether once they produced the required number of children.

These women end up consulting Dr Ng because they cannot handle their partners' sexual demands. 'She just cannot cope with it. She suffers from lack of interest. Why? Because she does not enjoy it - there's no orgasm.' Many Hong Kong women do not expect to enjoy sex because they have been raised to believe it is a messy, uncomfortable business that must be endured in order to reproduce.

Dr Ng said Viagra may make things worse for many couples. If men think they are on a wonder drug and demand more sex, the women for whom sex is an unpleasant marital duty rather than a pleasure may be unable to cope. 'Viagra will not help this problem. It will make it worse.'