A pill intended to prevent HIV infection kept more than 650 people HIV-free despite their high risk of infection, according to the findings of an observational study conducted in San Francisco. The vast majority of the individuals in the study were gay men, and many participants had multiple sexual partners. Despite a drop in condom use among those who took the pill, which is known as Truvada, none of the participants contracted HIV, based on information in their electronic health records.

Study participants had been taking Truvada from about mid-2012, when the drug was controversially approved by the U.S. Food and Drug Administration specifically for HIV prevention. Critics were skeptical that the pill, which needs to be taken daily, would fulfill the promise of early trials, and some warned that it could encourage individuals to engage in behaviors that increase their risk of HIV and other sexually transmitted diseases. Although some of those concerns remain, the new findings suggest that widespread HIV prevention among high-risk individuals may be possible with Truvada.