The drug, flibanserin, treats hypoactive sexual desire disorder in women. Sprout Pharmaceuticals said the FDA wants to see more data on how the drug interacts with other medications. Nearly 10 percent of women in trials reported sleepiness while taking the daily pill. (Allen G. Breed/Associated Press)

The Food and Drug Administration has approved 24 drugs for the treatment of male sexual dysfunction. For women, that number is zero.

The disparity reflects drugmakers’ difficulties in unlocking the secret to revving up women’s sex drive. It also has become a rallying point for women’s advocates and even some members of Congress, who suggest that federal regulators seem more eager to approve sex-enhancing drugs for men than for women.

It isn’t every day that political leaders and groups such as the National Organization for Women get involved in the drug-

approval process, particularly for “lifestyle” drugs. But the unsuccessful quest for a “female Viagra” is sparking complicated questions about a woman’s right to a pill that may improve her sex life — and about how much of a libido lift is enough to be judged effective by the FDA.

A drug called flibanserin, touted by some as the “little pink pill,” the counterpoint to Viagra’s little blue pill, was developed 12 years ago. Last month, women’s groups were disappointed when the FDA asked for further safety tests on the medication.

Some critics say the agency — consciously or not — may be succumbing to society’s squeam­ishness about women’s sexual desires compared with those of men.

“It looks to me like there are more hurdles being put in front of this drug than there have been on drugs addressing male dysfunction,” said Terry O’Neill, president of NOW. “Obviously, everyone only wants drugs to get on the market if they are proven safe and effective. But we don’t want attitudes to get in the way of a good drug.”

The FDA, which is led by a woman and has a woman at the helm of its Center for Drug Evaluation and Research, has denied the accusation. It stresses that it takes female sexual dysfunction seriously and is committed to working with companies trying to develop a marketable drug to treat it.

“We do not believe there has been any gender bias with regard to our review of this drug,” the agency said in a statement. “We engaged in a scientific process in which we evaluated whether the drug’s benefits outweigh its risk.”

Still, the agency’s reluctance to approve flibanserin, even as it signs off on new medications for men who struggle with intimacy, has gotten the attention of Congress. Last month, Rep. Debbie Wasserman Schultz (D-Fla.) and three other congresswomen sent a letter to the FDA urging “careful review employing the same standards of consideration given to approved drugs for men.”

A 2008 study suggested that 12 percent of women may suffer from female sexual problems. Drugmakers have been working for years to address hypoactive sexual desire disorder (HSDD), the most commonly reported type of female sexual dysfunction.

HSDD is defined as an absence of sexual fantasies and desire for sexual activity that does not have an obvious cause, such as depression or libido-suppressing drugs, and that causes distress to the sufferer.

People with the disorder often say they love and crave physical intimacy with their partners but are turned off by the idea of having sex with them.

“I have patients who call me who meet true criteria for HSDD, [asking] ‘Are we any closer? Is the drug out yet?’ ” said Leah Millheiser, director of the Female Sexual Medicine program at Stanford University. “There is a need for it.”

Low sex drive in women has been studied since the 1970s, but developing a treatment has proved more difficult than addressing male dysfunction, which often involves poor blood flow to the penis.

“The principal problem for men is plumbing. They don’t have low libido,” said Jan Shif­ren, director of the Midlife Women’s Health program at Massachusetts General Hospital. “Women don’t have a problem with anatomy. They have a much more complex problem.”

In her practice, she said, the reasons behind women’s low sex drive are often hard to determine. Her solutions range from urging a woman to go on a vacation with her partner to sending her to a physical therapist who specializes in pelvic-floor muscles. A lot of people want a pharmaceutical cure, she said, but she is skeptical.

“Everyone would like to find an answer in a medicine cabinet,” she said. “I suspect there will never be a simple pill.”

Sprout Pharmaceuticals, the small North Carolina drugmaker seeking approval for flibanserin, is the furthest along in the race to get such a drug on the market. Its drug tinkers with three brain chemicals believed to play a part in sexual arousal.

The FDA rejected flibanserin in 2010, saying it failed to meet effectiveness standards. The agency failed to approve it again last year, saying that research conducted since it was first rejected showed only “modest” benefits in increasing women’s sexual satisfaction.

But “our contention was, modest is still a meaningful effect,” said Cindy Whitehead, president of Sprout. “It doesn’t make a woman hypersexual or catapult her sex drive over her friends. But that was never the intent.”

Sprout appealed the FDA’s decision in December. Last month, the FDA requested three more safety tests, including one that looks at whether the drowsiness experienced by some women as a side effect impairs driving. The company said that it was “encouraged” by the response and that it will refile its application this fall.

The women’s groups say it should be up to women and their doctors, not the FDA, to decide if modest improvements in sexual satisfaction are good enough.

The issue caught the advocates’ attention in December, after Sprout filed its dispute. Without taking a position on the specific drug, the groups, including NOW, Jewish Women International and the National Council of Women’s Organizations, began questioning the FDA’s repeated rejections of flibanserin.

Issues related to sex and reproduction have long been part of the groups’ portfolios. But supporting a woman’s right to a fulfilling sexual experience is a new frontier.

“Rarely do we have an opportunity to have a public conversation about sexual pleasure and the right to a fulfilling sexual life,” said Serra Sippel, president of the Center for Health and Gender Equity, one of the groups that met with the FDA. “I think over the course of the next year, you’ll see more women’s reproductive health organizations joining this conversation.”