For 15 years, Carla Price and her husband's sex life was great. But then things began to change.

"Before, I would want to have sex," says Price, who is 50 and lives in central Missouri. "But over the years my sexual desire has just dwindled to nothing."

Price has no idea why. She's healthy. She's not really stressed out about anything. And she's still totally crazy about her husband.

"It's not that our relationship got boring," Price says. "Because it's actually the opposite — we became closer as we got older together."

But her lack of interest in sex almost wrecked their marriage.

"It did get to the point where my husband thought that perhaps we just needed to divorce," she says.

Women like Price, who see their decreasing sex drive as a problem, are at the center of an intense, emotional debate that's been raging for years over whether the Food and Drug Administration should approve the first drug that claims to boost a woman's libido.

NPR reached Price through Sprout Pharmaceuticals Inc., the company that makes the drug.

"Men have a number of treatment options for sexual dysfunction, says Cindy Whitehead, Sprout's CEO. "We haven't yet gotten to one for women's most common dysfunction."

"Up until now," she says, "the treatment paradigm for women with sexual dysfunction has essentially been: Let's take a drug that works in men and let's see if it works in women."

None of them did. But Sprout's drug, flibanserin, takes a totally different approach than, say, Viagra. Instead of increasing blood flow to the genitals, flibanserin affects a different part of the body: the brain.

Flibanserin shifts the balance of three key brain chemicals, Whitehead says. The drug, she says, increases "excitatory factors for sex" — dopamine and norepinephrine — and decreases serotonin, which can dampen the sex drive.

But there's a lot of skepticism about flibanserin. The FDA has rejected it twice, saying there wasn't much evidence it works. The agency also questioned the drug's safety, especially with long-term, daily use.

"The combination of ... not very robust effectiveness, and the fact that the safety profile had not been really characterized very well at all made us reach that conclusion, that it really wasn't ready for approval," says Sandra Kweder, deputy director of the FDA's Office of New Drugs.

The company acknowledges flibanserin can have side effects, including sleepiness, nausea and dizziness. And there are no results yet, Sprout says, on whether the drug might interfere with the helpful action of Zoloft, Prozac or other SSRI antidepressants, which are thought to work primarily by boosting levels of serotonin in the brain.

But Whitehead argues that flibanserin is safe and says the company's studies show it can help many women.

"We increase their desire by 53 percent," she says of study participants. "We decrease their distress by 29 percent, and then they doubled their number of satisfying sexual events."

Whitehead argues the FDA is holding flibanserin to a higher standard than it uses to evaluate drugs for men. And some women's rights advocates worry that might be true.

"We live in a culture that has historically discounted the importance of sexual pleasure and sexual desire for women," says Terry O'Neill, president of the National Organization for Women. "And I fear that it's that cultural attitude that men's sexual health is extremely important, but women's sexual health is not so important. That's the cultural attitude that I want to be sure the FDA has not, maybe unconsciously, imported into its deliberative process."

The FDA denies any bias.

"We have taken those concerns very seriously and we think the accusation is truly misplaced," Kweder says.

Many other women's health advocates agree with the agency's caution.

"It doesn't seem to work very well, if at all, and it's got some safety concerns that are troubling and haven't been fully explored," says Cindy Pearson of the National Women's Health Network. "So we felt very comfortable saying to the FDA, 'You know, women want attention, but they want drugs that work. And this doesn't seem to be one of them.' "

Others argue that the campaign for flibanserin is oversimplifying female sexuality. And many women (and men) who experience a waning libido at midlife don't see it as a problem.

"The misrepresentation that everybody should be having it — needs to have it, wants to have it, has a problem if they don't have it — is to change, really, what sexuality is into more of a medical thing," says Leonore Tiefer, a psychologist at New York University. "I think that's a terrible direction for knowledge, for understanding, for society."

Some say Sprout's campaign is part of a bigger trend by the pharmaceutical industry to turn everything into a disease that needs a pill.

"There's really been a move toward medicalizing normal human experience," says Adriane Fugh-Berman, who studies drug companies at Georgetown University. "And while there are certainly some women who have very troublesome symptoms of low libido, it's not at all clear that medication is a good answer for them."

A low libido may be a symptom of fluctuating hormones or of some health problem that needs attention. Some women may just be in a bad relationship. For others, therapy might be the answer.

Carla Price says she would like to try flibanserin. Marriage counseling and a hormonal cream have helped, she says. But not enough.

"Even though it's better, it's not perfect," she says. "I would gladly take risks of side effects to keep my marriage and my relationship."

Copyright 2016 NPR. To see more, visit http://www.npr.org/.