WASHINGTON — A pill that has long been used to treat HIV has moved one step closer to becoming the first drug approved to prevent healthy people from becoming infected with the virus that causes AIDS.

The Food and Drug Administration said Tuesday that Gilead Sciences’ Truvada appears to be safe and effective for HIV prevention. It concluded that taking the pill daily could spare patients “infection with a serious and life-threatening illness that requires lifelong treatment.”

On Thursday a panel of FDA advisers will consider the review when it votes on whether Truvada should be approved as a preventative treatment for people who are at high risk of contracting HIV through sexual intercourse.

The FDA is not required to follow the advice of its panels, but it usually does.

An estimated 1.2 million Americans have HIV, which attacks the immune system and, unless treated with antiviral drugs, develops into AIDS, a fatal condition in which the body cannot fight off infections.

If Truvada is approved, it would be a major breakthrough in the 30-year campaign against the AIDS epidemic. There have been no other drugs proven to prevent HIV, and a vaccine is believed to be decades away.

Gilead Sciences, based in Foster City, Calif., has marketed Truvada since 2004 as a treatment for people who are infected with the virus. The medication is a combination of two older HIV drugs, Emtriva and Viread.

Doctors usually prescribe the medications as part of a drug cocktail that makes it harder for the virus to reproduce. Patients with low viral levels have reduced symptoms and are far less likely to develop AIDS.

Researchers first reported that Truvada could prevent people from contracting HIV in 2010. A three-year study found that daily doses cut the risk of infection in healthy gay and bisexual men by 44 percent, when accompanied by condoms and counseling.

Another study found that Truvada reduced infection by 75 percent in heterosexual couples in which one partner was infected with HIV and the other was not.

Because Truvada already is on the market to manage HIV, some doctors already prescribe it as a preventive measure. But FDA approval would allow the Gilead Sciences to formally market its drug for that use.

FDA reviewers said patients must be diligent about taking the pill every day if using it as a preventive measure.