Trials have begun on a sex drug that works directly on the pleasure zones of a woman's brain to restore flagging libido.

If successful, flibanserin - developed by the German pharmaceutical company Boehringer Ingelheim - could become the "female Viagra".

Its effect was discovered by accident when it was being tested as an anti-depressant. Participants in the trials reported that their depression was no better but that they had experienced a boost in sexual desire.

The company is conducting four trials on 5,000 women in 220 locations and hopes for approval from the US Food and Drug Administration in 2009.

The company is not yet sure exactly how flibanserin works, but Dr Charles de Wet, Boehringer Ingelheim's medical director for the UK, said: "As many as two out of every 10 women describe some degree of decreased sexual desire. Female sexual dysfunction is not just related to blood flow, but also affected by stimulation of certain brain areas dealing with sexual stimuli."

He added: "The aim of flibanserin is to return women's sexual desire to a normal state - no excessive sexual effects have been reported from the treatment in any clinical trial."

Flibanserin pills stimulate parts of the brain associated with emotions and pleasure, including a circuit that appears to control desire and sexual arousal.

Viagra, which dominates the £1 billion worldwide market for men's sex drugs, is frequently used to increase the sexual enjoyment of men without a medical condition. Flibanserin takes several weeks for a significant effect to build up in the brain, meaning that, unlike Viagra, it cannot be "popped" in advance of an evening out.

Paula Hall, a sex therapist for the charity Relate, said she was doubtful whether such a drug would deal with the root problems of poor sex lives.

"The biggest cause of low sexual desire is relationship problems," she said. "Many women with low sexual desire have no problem with sexual excitement. The desire is there, but they are blocking it."

Shortcuts to arousal

A nasal spray drug called bremelanotide is intended to stimulate nerve pathways linked to sexual arousal and has the same effect on men and women

A drug named Intrinsa, developed by Procter & Gamble, is aimed at increasing female libido by boosting levels of the "male" hormone testosterone

Viagra, designed to help men with erectile dysfunction, was tested by Pfizer as a sex drug for women. It produced greater pelvic blood flow but failed to increase female libido