Clinical trials are being carried out on 200 women to test Viagra's impact on female sexual dysfunction. It is estimated that up to 40% of women suffer from problems with their sexual performance.

Dr Laura Berman: Sensation

The first preliminary results show that the drug, which works by increasing blood flow to the sexual organs, may have significant benefits.

Dr Laura Berman, a psychologist who is working on the trials, said: "We are finding that there is a significant increase in sensation, ability to become aroused and ability to achieve an orgasm in the women that were are treating."

The research, however, must take into account the fact that women's sexual function is more complicated than that of men's.

The BBC's Health Correspondent Karen Allen reports

Dr Berman's sister Jennifer, a urologist, said: "It is like apples and oranges. Although there are anatomic and embryologic parallels and physiologically blood vessels and nerves respond in a similar fashion, the context in which women and men experience sexuality is totally different."

Pfizer tests

Pfizer, the company that manufactures Viagra, is also carrying out research on the impact of the drug on women, including the possibility of combining it with hormone replacement therapy.

Michael Sweeney: New focus

Dr Michael Sweeney, Pfizer medical director, said: "It has been one of those areas that has not been a focus of research over the years.

"Women have not complained about it for societal reasons, and while women have not complained about it until recently then the impetus for science to investigate it has been much less."

"It will allow women to speak more freely and raise a problem that has been troubling them and to get treatment."

Sixty-one-year-old Evelyn is one of the women who is taking part in clinical trials of Viagra.

She said: "It is such a touchy and senstive area. To have things be better takes the emotional pressure off a relationship."