LONDON (Reuters Health) - Half of men taking Viagra can have sex within 20 minutes, Pfizer said on Monday as it fought off claims that its blockbuster impotence drug is slower-acting than rivals set to hit the market next year.

Eli Lilly & Co.'s drug Cialis, developed with US biotechnology company Icos Corp., has been widely reported to work faster than Viagra, but Pfizer said results of a new study showed its pill can also work fast.

In the double-blind study, 228 men previously successfully treated for mild, moderate or severe erectile dysfunction for at least two months were given stopwatches and randomly assigned to Viagra 100 mg or placebo for four weeks. The men, aged 27 to 82, recorded the time needed to obtain an erection hard enough for sexual intercourse after taking their pill and immediately beginning sexual stimulation.

Pfizer said results showed that within 14 minutes of taking Viagra, 35% of men achieved an erection that resulted in successful sexual intercourse. A majority of men, 51%, achieved this within 20 minutes.

The findings were due to be presented at the European Society for Sexual and Impotence Research in Hamburg.

Lead investigator Harin Padma-Nathan, from the University of Southern California, Los Angeles, told Reuters Health that he had also been principal investigator of one of the Cialis studies and was "very disturbed" by reports that it was faster acting than Viagra. Analysts had "completely misinterpreted the data," and Eli Lilly had not sought to correct the misperceptions, he said.

A Pfizer spokesman said Viagra dosage instructions suggesting men should take the pill about one hour before sex were based on the time the drug takes to reach maximum levels in the blood. However, this was different from the time it took to start working.

"With new products coming into the market place and talk about their more rapid onset of action, we decided to go back and look at this issue." The study results suggested a level playing field, he added.

Pfizer stressed that in any case fast-acting drugs may not be essential given the time men can take to act on their sexual impulses. It said that results of a survey of more than 1,000 men and women in Britain found that men with erectile dysfunction spend an average of 53 minutes from thinking about engaging in sexual activity to initiation of intercourse.

Sales of Viagra were roughly $437 million in the third quarter of 2002. Analysts estimate the impotence market will increase to between $3 billion and $4 billion a year by 2006 from $1.6 billion today.

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