The Food and Drug Administration says it has approved a new anti-HIV pill that combines four medicines to combat the virus in patients who have not previously been treated for infection.

The agency approved Gilead Sciences’ Stribild as a once-a-day treatment to control the virus that causes AIDS.

The pill contains two previously approved antiviral drugs 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 84 and 87 percent for patients taking older HIV drugs.