A new anti-impotence drug that works like Viagra but is effective for 24 hours is expected to reach Hong Kong within two years. The pill allows men to respond to sexual stimulation for up to a day after taking it. At present, Viagra users have to plan when they are going to have sex and then take their tablet at least an hour beforehand. The drug then works for between four and six hours. The new drug, called Cialis and developed by the makers of Prozac, is in the final stages of testing. A spokesman at manufacturer Eli Lilly's Shanghai office said it was expected to be available in Hong Kong within about two years. Cialis trials have so far involved more than 4,000 men. A meeting of the American Urological Association in California last weekend heard that 212 previously impotent men were able to have sex, on average, within 16 minutes of taking Cialis. They could still perform the next day, because the effects of the drug had not worn off. Side effects were reported by a minority of the men and were similar to those experienced by Viagra patients - headaches, aching muscles and facial flushing. Dr Andrew Yip Wai-chun, a consultant urologist at Kwong Wah Hospital, said a longer lasting impotence drug would be popular with patients as it would allow them to have a more normal sex life. 'If you take a drug for sex, you really don't want to be bound by a time-frame, so if you take a drug that's got a longer period of effectiveness, you feel more at ease,' he said. 'Rather than feeling that you have to be in a hurry, . . . people will be more relaxed and they can enjoy sex more.' Viagra sales last year hit $10.5 billion worldwide. Manufacturer Pfizer will not say how many pills are sold in Hong Kong, where the cost of a four-pill pack ranges from about $320 to $350. Eli Lilly said it was not yet known how much Cialis would cost in the SAR. Viagra has been available under doctors' prescription in Hong Kong since February 1998. Dr Yip said that in the past two years, Kwong Wah Hospital had prescribed it to more than 5,000 patients. He said the men's clinic used to see between 800 and 900 patients a year. Since Viagra that number had soared to 2,500 as the ease of treatment encouraged many men who were previously reluctant to get help to come forward. About 10 per cent of Hong Kong men are estimated to suffer from erectile dysfunction. The South China Morning Post reported in February that three Hong Kong men had suffered heart attacks induced by Viagra. Two of the men had bought the drug illegally. The third was prescribed it by his family doctor. All three survived following surgery for blocked arteries caused by taking the drug while suffering from undiagnosed heart conditions. Viagra and Cialis are unsuitable for patients with certain heart conditions and for those taking nitrates, often used to treat heart problems.