Results of a study published this week show that a daily dose of the antiretroviral drug Truvada reduced the risk of acquiring HIV infection by 44 percent in men who have sex with men.

The research may provide HIV-negative men with an additional method for safeguarding themselves from HIV infection. The findings could also be important for men in serodiscordant relationships, in which one partner is HIV positive and the other is HIV negative.

“We now have strong evidence that pre-exposure prophylaxis with an antiretroviral drug, a strategy widely referred to as PrEP, can reduce the risk of HIV acquisition among men who have sex with men, a segment of the population disproportionately affected by HIV/AIDS,” said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, in a press release.

Approximately 5 to 10 percent of all HIV infections that occur worldwide stem from men who have sex with men. In the United States, men who have sex with men account for more than half of all new HIV infections every year.

Pre-exposure prophylaxis is a practice in which HIV-negative individuals are given antiretrovirals prior to being exposed to the virus in the hopes of preventing infection from occurring.

The idea for pre-exposure prophylaxis comes from results showing that antiretrovirals successfully reduce the chance of HIV transmission from pregnant women to their babies by about 10-fold. Several large studies, including this one, are attempting to determine whether antiretrovirals may be similarly effective in preventing HIV transmission in general.

In this study, the researchers studied 2,499 men who have sex with men. The men were recruited from 11 different sites in Brazil, Peru, Ecuador, Thailand, South Africa, and the U.S.

Each participant was randomly assigned to receive either a once-daily pill of the anti-HIV medication Truvada (emtricitabine/tenofovir) or a placebo pill. All participants were also instructed to take normal safe-sex precautions against infection.

Participants were monitored with regular HIV tests for a median of 1.2 years.

Results showed that taking antiretrovirals reduced the risk of contracting HIV by 44 percent. By the end of the study, 36 (3 percent) of the 1,251 study participants who received antiretroviral therapy became infected with HIV. Of the 1,248 participants who received the placebo pill, 64 (5 percent) became infected with HIV by the end of the study.

Furthermore, participants who strictly followed the treatment regimen and took the medication at least 90 percent of the time reduced their risk of acquiring HIV by 73 percent.

The most common side effects of the regimen included nausea (2 percent of participants receiving Truvada) and weight loss of 5 percent or more (3 percent).

The researchers concluded that once-daily Truvada successfully decreased the risk of acquiring HIV in men who have sex with men.

According to Dr. Fauci, “Additional research is needed, but certainly this is an important finding that provides the basis for further investigating, developing and employing this prevention strategy, which has the potential to make a significant impact in the fight against HIV/AIDS.”

A follow-up study, which will last for an additional 18 months, will provide more information on the regimen’s long-term safety and effectiveness in preventing HIV transmission.

The researchers noted that further study is necessary to assess whether similar prevention methods could be equally successful in other populations, such as in women or heterosexual men.

For further information, please see the New England Journal of Medicine or the press release from the National Institutes of Health.