On Friday the FDA gave its approval to an impotence remedy called Cialis from pharmaceutical company Eli Lilly and biotechnology firm Icos Corporation.

The drug is set to rival Viagra, which men are advised to take one hour before sex and which lasts up to four hours.

Cialis, already approved in Europe, is said to last 24 hours and should be available in the US within 10 days.

There's a huge opportunity in the US market Khoso Baluch, Eli Lilly

While the effects of the drug last all day, men will only achieve an erection when they are sexually aroused.

The market for erectile dysfunction drugs is becoming increasingly competitive following the success of Pfizer's Viagra, which hit the US market in 1998, and had sales of about $1.7 bin last year.

'Normal lives'

In August, UK-based GlaxoSmithKline and German-based Bayer won FDA approval for their anti-impotence drug, Levitra.

GlaxoSmithKline and Bayer have mounted an aggressive marketing campaign that includes a sponsorship deal with the National Football League to promote Levitra.

Eli Lilly and Icos have a sponsorship deal with the PGA, the association for professional golfers, to promote Cialis.

Khoso Baluch, of Eli Lilly, said:"There's a huge opportunity in the US market.

"There are 27 million men not getting treatment, and Cialis opens the window for them to get their lives back to normal."