Viagra, a new impotence pill, has been on the shelves for less than ten days, but doctors cannot seem to write prescriptions fast enough. They are being besieged by hundreds of thousands of impotent men, not to speak of normal men seeking to enhance sexual performance. According to some estimates, some 40,000 prescriptions are being written every day.

Pharmacy industry experts estimate that at $ 10-a-pill, Viagra is set to gross more than $ 5 billion a year in the United States alone, if one takes a conservative estimate of five million men using the pill twice a week. Industry thumb rule deems any drug that brings in $ 1 billion a year worldwide to be a bestseller. The share price of Pfizer Inc., the drug's manufacturer, soared to a record $ 121.75 as prospectorsgrabbed stock. Riding on the Viagra hype, Pfizer's stock has climbed 30 per cent since early March, making it the top pharmaceutical company ahead of E.Merck.

The first oral pill to treat impotence, Viagra was licensed by the US Food and Drug Administration on March 27. Unlike previously approved treatments for impotence, Viagra does not directly cause penile erections, but affects the response to sexual stimulation. The generic name for Viagra is sildenafilcitrate. According to the FDA, the drug acts by enhancing the smooth muscle relaxant effects of nitric oxide, a substance that is normally released locally in response to sexual stimulation. This smooth muscle relaxation allows blood to enter and pool in the penis leading to an erection.

The FDA clearance says Viagra is best taken one a day about an hour before sexual activity. It is generally well tolerated by the system but side effects could include headache, flushing, upset stomach, urinary tract infection and diarrhoea. Viagra's launch was prettylow key and there is still hardly any advertising. But media coverage and word of mouth has more than made up for it.So furious is the demand that urologists are now establishing separate Viagra hours, automatic telephone switchboards are being reworked (`Press 3 for Viagra'), and pharmacies have been forced to make wait lists.

``In the 20 years I've been involved in urology, I've never seen anything like it. If we were in the military, I think we would call in and say our position is being overrun,'' Dr James B. Regan, the head of Georgetown's Erectile Dysfunction Treatment Program, was quoted as saying in the local media.

What is driving the demand for Viagra is not just the demand from sexually dysfunctional men but also men wanting to try the drug to see if it improves their normal performance. Doctors say there is no evidence it does, and it could, in fact, pose unknown health risks. They say it works best on men who are basically impotent.

Even at $ 10 a pill, doctors say it is set to emerge asthe most preferred option to beat impotence. Other current tested methods like penile implants involve surgery costing $ 15,000 to $ 20,000. Expectedly, Viagra's rousing success has led to a cottage industry of clones and imitators. A herbal concoction called Viagro has popped up on the Internet and an Atlanta company is selling a sound-alike pill called Vaegra. Overnight, sex magazines and phone sex lines have begun to peddle assorted, similar sounding pills.