When Pfizer launched Viagra in 1998 its share price doubled within days. Since then, the little blue pills have become a pillar of profit, earning the company more than £1bn a year.

Thanks to promotional campaigns, which included appearances by the footballing legend Pele, male impotence lost some of its stigma and 25 million men requested the pills. In England alone, GPs write about 6m Viagra prescriptions a year.

The drug started life in a lab in Sandwich, Kent, where it was developed to treat high blood pressure. Its transformation into a blockbuster treatment for impotence began when volunteers in a clinical trial reported a suspicious number of erections. The overnight success of Viagra prompted Pfizer to wonder if the drug had any effect in women. They raised awareness of a condition called "female sexual arousal disorder", an all-encompassing phrase for sexual dysfunction, and began clinical trials. The trials were a failure and the attempt to have Viagra licensed for the condition was abandoned.

Pfizer has been criticised for overstating the benefits of Viagra. It claims "more than half of all men over 40 have some difficulty getting and maintaining an erection". In 2004, the US Food and Drug Administration forced Pfizer to pull a series of advertisements because it made unsubstantiated claims about the drug's effectiveness.

Some psychologists warn Viagra has become a lifestyle drug that encourages people to neglect underlying mental or physiological problems that can cause impotence. The anti-obestity drug, orlistat, came under fire for similar reasons. Critics said it fostered the misconception that modern ills can be dealt with by a pill instead of living a healthier life.