FDA approved HIV-prevention drug Truvada blocks an enzyme in the body that the HIV virus needs to proliferate.

Justin Sullivan/Getty Images

n a breakthrough moment in the fight against HIV, the Food and Drug Administration on Monday granted its approval to Truvada, the first medication that helps prevent the spread of the virus among people having sex with infected partners. Here's what you need to know:

How does it work?
Truvada is a combination of two different medicines: Emtricitabine and tenofovir disoproxil fumarate. The drug works by blocking an enzyme in the body that the HIV virus needs to proliferate.

How effective is it?
It's not perfect, but it certainly helps. In one trial — in which an HIV-negative individual had unprotected sex with multiple partners (some of whom were HIV-positive) — the drug reduced the risk of contracting the virus by 42 percent compared to a placebo. In another trial for heterosexual couples in which one partner was infected and condoms were regularly used, Truvada reduced HIV infections by up to 75 percent. That means the pill works best when used in conjunction with other prevention methods. "It's not just, 'Here's a pill, take it, and you're rendered protected," researcher Dr. Kenneth Mayer of Fenway Health in Boston tells NPR. It's most effective when used responsibly, and that means also employing contraceptives, risk reduction counseling, and regular HIV testing.

Who should take the pill?
For "healthy, uninfected people, the drug can thwart HIV's ability to take hold in healthy cells and start an infection," says Alice Park at TIME, but it won't help anyone who's already infected. Still, the pill, which is meant to be taken daily, could be hugely important for the estimated 415,000 Americans whose sexual activity puts them at the highest risk of contracting HIV, says Victoria Colliver at the San Francisco Chronicle. Every year, 50,000 U.S. adults and adolescents are diagnosed with the disease, which affects a total of 1.2 million across the country.

How much does it cost?
If you take Truvada every day (as recommended), it will likely cost in the neighborhood of $14,000 per year.

Are there side effects?
The most common side effects include diarrhea, nausea, abdominal pain, headache, and weight loss. In rare cases, the drug caused kidney problems or bone toxicity. There's also the risk of the HIV virus developing an immunity to Truvada, which is why regular testing is important. Some critics also worry that the drug could lead infected patients to engage in riskier sexual behavior.

Sources: Boston Globe, NPR, San Francisco Chronicle, TIME