WASHINGTON - A drug that failed to fight the blues could be the female answer to the little blue pill Viagra, the lead North American investigator analysing tests of the drug said Tuesday.

Women who took the drug flibanserin when it was being tested as an anti-depressant said it didn't help them beat the glums, but did give them "an increase in libido that they liked," John Thorp told AFP.

Lack of desire is the most common sexual problem in women aged 30 to 60, just as erectile dysfunction, for which Viagra is one of a choice of treatments, is the most common sexual disorder among men in the same age bracket, Thorp said.

"Men remain interested but can't act or perform properly and women lose interest," said Thorp, one of the investigators analyzing data from three clinical trials of the drug.

"So where Viagra and other erectile dysfunction medications work in the blood supply, flibanserin works in the brain," he said.

In light of the women's reactions to flibanserin, the German drug company that first tested the drug as a treatment for depression, Boehringer Ingelheim, several years ago began exploring the possibility of it being the female answer to Viagra.

Ironically, flibanserin and sildenafil, the active ingredient in Viagra, were both being tested as treatments for another condition when they were found to have their happy sexual side-effects.

Sildenafil was being tested by scientists at Pfizer Laboratories as a cardiovascular drug and for its ability to lower blood pressure.

But during the tests, researchers found that "people didn't want to give the medication back because of the side effect of having erections that were harder, firmer and lasted longer," Dr Brian Klee, senior medical director at Pfizer, told AFP.

Clinical trials to test flibanserin's efficacy in raising the level of sexual desire in women were held in Canada, Europe and the United States.

Nearly 2,000 pre-menopausal women were given flibanserin or a placebo for 24 weeks and asked to report back to researchers or make diary entries on six variables, including the number of satisfactory sexual encounters they had and their level of sexual desire.

The studies found that 100 milligrams a day of flibanserin resulted in "significant improvements" in the two variables.

Flibanserin is currently an investigational drug and is only available to women taking part in clinical trials, which may be depressing news for the up to 26 percent of American women who report diminished sexual desire, according to Thorp.