The active ingredient in Propecia started life as a treatment for enlargement of the prostate gland. Scientists at the pharmaceutical company Merck became interested in it as a baldness drug when men taking it for their prostate started noticing that their hair was growing back.

Official approval for a low-dose version of the drug to be used specifically for hair loss has come on the basis of three year-long studies of nearly 2,000 mild to moderately bald men. In men taking the pills, 83% stopped losing hair, compared to less than 30% of men who were given dummy tablets.

Some men taking Propecia even began to regain their hair. The drug works against the male hormone testosterone, which is linked with male pattern baldness.

It's this anti-male hormone action that also makes it useful against prostate disease - and which accounts for its side effects. According to the manufacturers, nearly 2% of patients experienced decreased sexual desire, or had difficulty in achieving an erection.