The U.S. Food and Drug Administration on Monday approved a new anti-HIV pill that combines four medicines to combat the virus that causes AIDS.

The agency approved Gilead Sciences’ Stribild as a once-a-day treatment to control HIV in adults who have not previously been treated for infection.

The pill contains two previously approved antiviral drugs, emtricitabine and tenofovir disoproxil fumarate, currently sold as the combination pill Truvada. Those drugs are combined with two new drugs: elvitegravir and cobicistat. Elvitegravir interferes with one of the enzymes that HIV needs to multiply. Cobicistat helps prolong the effect of elvitegravir.

Company studies showed that 88 to 90 percent of patients taking Stribild had an undetectable level of HIV in their blood after 48 weeks, compared with 87 percent for patients taking Atripla, another HIV drug that contains Truvada and one other drug.

An estimated 1.2 million Americans have HIV, which develops into AIDS unless treated with antiviral drugs. AIDS causes the body’s immune system to break down, leading to infections which are eventually fatal.

Patients can live relatively healthy, normal lives when treated with antiviral cocktails.

Like most other HIV drugs, Stribild will carry a boxed warning about potentially dangerous side effects, including severe liver problems and the buildup of lactic acid. More common side effects include nausea and diarrhea.

Earlier this year, Gilead received FDA approval to market Truvada as the first preventive medicine for healthy people who are at high risk of acquiring HIV. Truvada was first approved in 2004 for patients already infected with the virus.

Gilead Sciences Inc. is based in Foster City, Calif. Shares of the company fell 10 cents to close at $57.19 and then added 11 cents in after-hours trading.