The 15-year 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 has reached an impasse with the US Food and Drug Administration over its drug, flibanserin. The daily pill was designed to increase libido in women by acting on brain chemicals linked to mood and appetite.
The FDA questioned whether the drug's benefits outweighed its risks, considering its ''modest'' effectiveness and side effects including fatigue, dizziness and nausea.
Sprout said it was appealing an October letter from the FDA that denied approval and asked for more information. But chances for approval appeared slim: Of the 17 appeals FDA considered last year, 14 were denied, according to government figures.
The agency's letter was the latest challenge for companies working to develop therapies for women who reported stress due to lack of libido.
It's a market drugmakers have been trying to tap since the success of Viagra, an erectile-dysfunction drug that was developed in the late 1990s to increase blood flow to the genitals.
But unlike sexual problems in men, most of women's sexual issues were psychological, not physical. As a result, there were 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. Likewise, other factors must be considered when treating it.
Experts said that developing drugs for female sexual dysfunction was so difficult because of how little we understood the underlying causes.
''Erectile dysfunction is a really easy thing to measure,'' said 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, said the idea that a single pill could restore female libido oversimplified the problem.
Even if the FDA eventually approved a drug for female sexual dysfunction, she said it would likely be used with non-drug techniques to reduce stress and improve self-image.
''A pill just doesn't take care of it,'' said Sadock, who taught human sexuality at New York University's School of Medicine.
''You may take a statin drug to control your cholesterol, great. But you should also exercise and you should also watch your diet.''
Drugmakers have made several unsuccessful attempts at tweaking their approach to boosting female libido over the years.
Initially, Pfizer tested Viagra on women, hoping that the drug's ability to increase blood flow to genitals would increase sex drive in women.
When that didn't work, drugmakers turned to hormones, including the male hormone testosterone.
In 2004, an FDA panel rejected Procter & Gamble's testosterone patch, Intrinsa, due to questions about its long-term safety - despite evidence of effectiveness.
Sprout's flibanserin was the first drug to approach the problem through brain chemistry.
Sprout acquired flibanserin from Boehringer Ingelheim in 2011, after the German drugmaker abandoned development of the pill following an FDA rejection letter.
Researchers believed the drug worked by boosting dopamine - a brain transmitter associated with appetite - while lowering serotonin - another transmitter linked with feelings of satiation.
Studies of the drug showed that it boosted sexual desire, reduced stress and increased ''sexually satisfying events,'' reported by women with hypoactive sexual desire disorder, or a lack of sexual appetite that caused stress.
The FDA has twice rejected flibanserin since 2010. A key issue for the agency is that women taking the drug reported only 1.7 more satisfying sexual experiences per month than women taking placebo.
Sprout executives argued that number was statistically significant and warranted approval for their product, considering there were no other drugs approved for the condition.
''We've now got 24 drugs for men for either testosterone replacement or erectile dysfunction,'' said Cindy Whitehead, Sprout's chief operating officer.
''Yet there are zero drugs for the most common form of sexual dysfunction in women.''
The FDA, which does not comment on drugs under appeal, was expected to make a decision on Sprout's appeal in the first quarter of next year.