U.S. health regulators on Monday approved Gilead Sciences Inc's four-drug combination pill to treat HIV, the virus that causes AIDS.

The four drugs in one pill, which was formerly called the Quad and had been considered one of Gilead's more important future growth drivers, will be sold under the brand name Stribild, the U.S. Food and Drug Administration said.

Gilead shares were virtually unchanged from their $57.19 Nasdaq close as Wall Street has been focused on development of the experimental hepatitis C treatment that Gilead acquired with its $11 billion purchase of Pharmasset.

"The approval was on time as expected," said RBC Capital Markets analyst Michael Yee, adding the he expects the new pill will meet or exceed current Wall Street estimates for sales of about $46 million this year and $370 million next year.

Yee said he expects Gilead to aggressively market Stribild and deemphasize its Atripla and Complera combination therapies as Atripla contains a Bristol-Myers Squibb Co HIV drug and Complera contains a Johnson & Johnson drug.

"The profit margins are significantly better because this drug is wholly owned by Gilead," Yee said of the quad pill.

Stribild, a once-daily treatment, contains two previously approved HIV medicines and two new drugs. The older medicines, emtricitabine and tenofovir, are currently sold by Gilead in its combination pill Truvada, which was approved in 2004, and are also part of Gilead's Atripla three-drug combination. The newer drugs are elvitegravir and cobicistat.

"Through continued research and drug development, treatment for those infected with HIV has evolved from multi-pill regimens to single-pill regimens," Edward Cox, director of the Office of Antimicrobial Products at the FDA's Center for Drug Evaluation and Research, said in a statement. "New combination HIV drugs like Stribild help simplify treatment regimens."

As a condition of approval, Gilead will be required to conduct additional studies to help further characterize the drug's safety in women and children, how resistance develops to Stribild, and the possibility of interactions between Stribild and other drugs, the FDA said.

The drug's label will also carry a boxed warning of the potential for a build up of lactic acid in the blood and severe liver problems, both of which can be fatal. The warning also states that Stribild is not approved to treat chronic hepatitis B virus infection, the FDA said.

About 1.2 million people in the United States are infected with HIV, according to the U.S. Centers for Disease Control and Prevention.