A team at clinic at Columbia Presbyterian Centre in New York, studied the impact of the drug on women.

Dr Steven Kaplan, a urologist at the clinic, said that about a quarter of the patients had some improvement in overall sexual function.

But he said a similar result was found among men who took a placebo instead of Viagra itself.

He said: "We found that there was no significant change either in intercourse satisfaction or in the degree of sexual desire after the patients had taken Viagra for 12 weeks."

Dr Kaplan's study, published in the journal Urology, is the first published study of Viagra in women, although several others are being conducted.

Viagra, known generically as sildenafil, works by increasing the effects of nitric oxide, a common body chemical, which in turn gets more blood flowing into the genitals.

Doctors have said theoretically it should work the same way for women - helping sexual function once a person is aroused.

Female impotence

Manufactuerers Pfizer says seven million prescriptions have been written for Viagra worldwide.

It is carrying out its own trials on women who have a problem reaching orgasm.

It is estimated that up to one in 10 men in the UK are impotent. The numbers of women who are unable to have an orgasm due to clitoral erection is unknown since women do not traditionally complain about sexual problems.