Viagra has proven to be hugely popular

Levitra, the first market rival to the hugely profitable Viagra, is being manufactured by the German firm, Bayer AG, and marketed by the British firm GlaxoSmithKline (GSK) as an alternative oral therapy for erectile dysfunction.

Since its appearance in 1998, sales of Pfizer's Viagra have risen to nearly $2bn a year.

Both pills work in the same manner and doctors warn that both can have serious side effects, especially for men with heart problems.

A third anti-impotence pill, Cialis, is expected to reach the US market later this year.

The US Food and Drug Administration (FDA) approved Levitra, an orange pill compared to Viagra's blue, based on studies showing that men were on average five times more likely to achieve an erection suitable for intercourse when taking the pill compared with those given a dummy medicine.

Researchers reported that studies of several thousand men showed that Viagra helped more than 70% improve their erections.

As well as the warnings to men with heart conditions, the FDA said Levitra was not for patients with who had suffered a recent heart attack or stroke who have very low blood pressure or uncontrolled high blood pressure.

For otherwise healthy men, Levitra's main side effects were headache, flushing and a stuffy nose and sometimes dizziness.

Lawson Macartney, head of strategic management of GlaxoSmithKline's cardiovascular, metabolic and urology drugs, said: "We know, from considerable market research, that the market is ready for new options".