Though both medications have taken their turns being dubbed the "female Viagra" in media reports, they are very different from the little blue pill. Viagra helps men who already want to have sex but are not able to physiologically. It is taken on an as-needed basis to help increase blood flow to the penis.

In contrast, Addyi changes brain chemistry in women to help them want to have sex and must be taken every day. Vyleesi, while taken on an as-need basis, is an injection that targets brain hormones to try and change a woman’s sexual desire.

The participants who enrolled in the initial clinical trials for Addyi were premenopausal, heterosexual, and generally healthy women. In four of the trials, an overwhelming majority of these participants identified as White (89%). In three trials, 78% were married, 74% of participants had never smoked. For Vyleesi, 86% of participants were White and those taking common blood pressure or depression medications were excluded from a study. Given the general exclusion criteria and demographic information for the trials, the extent to which the results can be generalized to the public is limited.

After adjusting for placebo, an FDA review found only 8-13% of women in clinical trials benefited—and minimally—from taking Addyi. For this small subset of women, the minimal benefits of taking Addyi were felt by about 8 weeks. This means women shouldn’t keep taking Addyi beyond 8 weeks if they do not experience an improvement in sexual desire by that point.

Similarly, in clinical trials for Vyleesi, a mere 8% of women experienced an increase in sexual desire as a result of the drug.

Addyi can depress the central nervous system. During clinical trials, women who used Addyi were more likely to experience dangerously low blood pressure, dizziness, and loss of consciousness than women taking placebo. These studies also revealed a higher incidence of accidental injury in women taking Addyi.

For women taking Vyleesi, nausea was the most common adverse effect. Thirteen percent (13%) needed medications for the treatment of nausea. Other effects include facial, gum, or breast darkening. Vyleesi can also raise someone’s blood pressure but lower their heart rate.

Women may experience fatigue, low blood pressure, and fainting when using Addyi, all conditions which may be exacerbated when drinking alcohol. As of April 2019, the FDA advises women to stop drinking alcohol two hours before their daily bedtime dose of Addyi and to not consume alcohol until 8-10 hours later based on post-marketing studies run by the company. The FDA considers this risk important enough to mandate a black box warning about potentially dangerous low blood pressure and fainting, especially when taken with alcohol.

In a small study, taking Vyleesi at the same time as drinking alcohol (the equivalent to 3 glasses of wine) didn’t cause any ill effects.

Using Addyi while using hormonal contraceptives may cause additional side effects.

Using Addyi while on hormonal contraceptives may cause additional side effects. Some adverse events, including sleepiness, dizziness and fatigue, were reported more often in women who used hormonal contraceptives. Hormonal contraceptives are known to affect a certain enzyme (CYP3A4), which may increase Addyi exposure by 40%.

Women taking hormonal contraceptives together with Vyleesi didn’t see a statistically meaningful rise in sexual desire. However, when taking Vyleesi, women’s bodies still absorbed common contraceptive hormones in a typical way.

Many commonplace medications are CYP3A4 inhibitors and may significantly increase a body’s exposure to Addyi. This could lead to poor tolerability of the drug, as well as increases in low blood pressure and fainting for women using Addyi. These medications include antiepileptic drugs, benzodiazepines, antidepressants, antipsychotics, mood stabilizers, narcotics, vaginal lubricants, and even St. John’s wort, all of which were excluded from initial studies.

Vyleesi interacts with everyday treatments for those struggling with substance use disorder (e.g. naltrexone), and may slow down the absorption of other drugs, making them ineffective (e.g. antibiotics, pain medications).

These two drugs "work" by changing brain chemistry to affect desire for sex, or libido, in premenopausal women. Neither is designed to make achieving an orgasm easier nor do they necessarily make sex more enjoyable for either partner.

In 2016, the FDA published soft-ball draft guidance for companies developing sex drugs for women. The guidelines recommend that companies test different doses, justify each distinct drug component, create interpretable study results, and complete adequate safety assessments. The FDA no longer requires companies to show that their sex drugs for women result in more sex – perhaps because no drug reviewed by the FDA has been able to pass that test.

The FDA does not challenge or discuss whether sexual dysfunction is as widespread as manufacturers claim and whether a drug is the best way to address low libido in women.

There are many unresolved questions about the seriousness, severity, duration, and frequency of side effects from Addyi and Vyleesi. With good information, women can make the good decisions about their health care. Currently, women have to decide whether or not to use these drugs without all the information they deserve.