The U.S. Food and Drug Administration is expected to announce its ruling on a controversial new drug called Addyi on Tuesday. If approved, the little pink pill will be the first drug on the market to boost a woman’s sex drive and could soon find its place in the medicine cabinets of millions of American women alongside Tylenol -- and their husbands’ Viagra.

Though Addyi, also known as flibanserin, has been called “female Viagra,” it has little in common with the erectile dysfunction drug. Viagra boosts blood flow to a man’s penis to make it erect once he is already aroused. But Addyi aims to put women in the mood in the first place by treating a little-known condition called hypoactive sexual desire disorder (HSDD).

HSDD is defined as a sudden and persistent loss of sexual desire that is not due to relationship issues, stress or life circumstances and which causes anxiety and distress in a patient’s life. Women with HSDD are not asexual -- they harbored a stronger sex drive in the past and may still enjoy sex once they start having it, but no longer feel the desire to initiate intercourse. HSDD is typically diagnosed in women who are in long-term, stable relationships.

It’s not entirely clear to scientists why some women experience such a drop in desire. Brain scans of women with HSDD show a difference in the way these patients react to erotic images as compared with healthy women, but researchers aren’t sure what neurological mechanisms are to blame for that switch.

It’s also not clear how Addyi, which was originally developed as an antidepressant, boosts desire. When the drug failed to improve the mood of depressed patients in clinical trials, researchers noted that it did seem to increase the number of sexual thoughts that these patients experienced, and switched gears to develop the drug for women with HSDD. But they still can't explain why or how the drug works. 

Here’s what scientists do know: Addyi adjusts levels of dopamine and two types of serotonin in the brain. Researchers suspect these neurotransmitters play a role in regulating hormones that affect sexual desire. While anti-depressants tend to increase the amount of serotonin in the brain, which can lessen desire, Addyi likely enhances the release of dopamine instead.

If approved, the drug could be in high demand. HSDD is considered the most common form of female sexual dysfunction, which is the FDA's official term for a group of related disorders that have to do with sexual arousal and desire in women. Collectively, these disorders are classified as an area of unmet medical need. Sprout Pharmaceuticals, the company behind Addyi, estimates that 16 million American women, or 1 in 8, suffer from HSDD. 

But since there are many ways to define sexual desire, diagnosing a patient with HSDD is a somewhat subjective decision. Doctors first ask patients about their recent history of childbirth, relationship problems and other factors that are known to cause a temporary drop in sexual desire. If a patient has no such history, the physician can proceed with the diagnosis.

Next, the physician confirms with the patient that they are in fact bothered by their loss of sexual desire by asking if it’s a source of anxiety for them. If a woman and her partner are fine with not wanting or having sex, there’s no reason to change that. Some doctors use a brief survey, called the Decreased Sexual Desire Screener, which lists questions such as, “Would you like your level of sexual desire or interest to increase?” to identify symptoms and rule out other potential causes of low desire in patients.

Of course, there are many reasons why a woman may have a low sex drive that are wholly unrelated to hormones or neurons. Some women experience dryness following menopause which makes sex uncomfortable. Women with hypersensitive genitals feel intense pain during penetrative sex. Since Addyi is only meant to be prescribed to premenopausal HSDD patients, it will not help many women who suffer from low desire for other reasons.