Public-health officials are pushing for much wider use of a drug that has proved effective at preventing HIV infection.

PrEP, or preexposure prophylaxis, is a daily medication that people at high risk for HIV can take to protect against acquiring the virus, which causes AIDS. The drug Truvada was approved by the Food and Drug Administration in 2012 for use as PrEP. About 50,000 people a year in the U.S. become infected with HIV, according to the Centers for Disease Control and Prevention.

Still,...