Sexual difficulties in women are very complex

Dr Rosemary Basson, lead researcher

The women took either 10, 50 or 100 milligrams of Viagra one hour before sexual activity for 12 weeks.

The researchers found that Viagra did not make any significant difference.

In fact, they found up to 50% of the women studied felt their sexual performance had improved after they had taken a placebo..

Head researcher Dr Rosemary Basson said the results "confirmed that sexual difficulties in women are very complex".

Viagra does increase blood flow to the woman's genital area - but the study found that this did not necessarily lead to greater sexual arousal.

'Don't abandon Viagra'

We need to do a heck of a lot better job in defining female sexual dysfunction

Dr Steven Kaplan, Columbia University

Dr Steven Kaplan, a professor and vice chairman of urology at Columbia University in New York, said that women should not be discouraged and that Viagra may work for some of them.

Dr Kaplan carried out a smaller study last year that came to the same conclusion.

But he said: "We need to do a heck of a lot better job in defining female sexual dysfunction.

"I would not abandon Viagra in women."

Pfizer has begun another study in the US focusing on post-menopausal women who are receiving hormone replacement therapy to see if Viagra helps them.

Viagra has helped several women to become pregnant after suffering years of infertility.

It stimulates a thickening of the lining of the uterus, so that it is able to provide effective nutrition to a developing embryo.

The research was presented at a meeting of the American College of Obstetricians and Gynecologists.