The multi-decade search for a pill that boosts sexual desire in women has hit another roadblock, raising questions about the future of efforts to develop a female equivalent to Viagra.
Sprout Pharmaceuticals said Wednesday it has reached an impasse with the Food and Drug Administration over its drug, flibanserin. The daily pill is designed to increase libido in women by acting on brain chemicals linked to mood and appetite.
The FDA questions whether the drug's benefits outweigh its risks, considering its "modest" effectiveness and side effects including fatigue, dizziness and nausea.
Sprout said it is appealing an October letter from the FDA that denied approval and asked for more information. But chances for approval appear slim: Of the 17 appeals FDA considered last year, 14 were denied, according to government figures.
(Read more: The next big sex drug? Clinical trials start soon)
The agency's latest rejection raises serious questions for more than a half-dozen companies working to develop therapies for women who report stress due to lack of libido. It's a market drugmakers have been trying to tap since the blockbuster success of Viagra, an erectile-dysfunction drug approved in the late 1990s to increase blood flow to the genitals.
But unlike sexual problems in men, most of women's sexual issues are psychological, not physical. As a result, there are a number of alternate causes doctors must consider before diagnosing female sexual desire disorders, including relationship problems, hormone disorders, depression and mood issues caused by other medications.
Experts say that developing drugs for female sexual dysfunction is so difficult because of how poorly we understand the root causes.
"Erectile dysfunction is a really easy thing to measure," says Emory University researcher Kim Wallen. "Motivation is a hard thing to measure and, quite honestly, we don't know enough about what creates sexual motivation to manipulate it."
And Dr. Virginia Sadock, a psychiatrist, says the idea that a single pill can restore female libido oversimplifies the problem. Even if the FDA eventually approves a drug for female sexual dysfunction, she said, it will likely be used with nondrug techniques to reduce stress and improve self-image.