After years of debate on this question, a new international study suggests the medication doesn't lead people to stop using condoms or have more sex with more people.
The research isn't definitive, and it hasn't changed the mind of every expert. But one of the study's co-authors said the findings support the drug's use as a way to prevent infection with HIV, the virus that causes AIDS.
"People may have more partners or stop using condoms, but as well as we can tell, it's not because of [taking the drug to prevent HIV infection]," said study co-author Dr. Robert Grant, a senior investigator with the Gladstone Institute of Virology and Immunology in San Francisco.
The medication in question is called Truvada, which combines the drugs emtricitabine and tenofovir. It's normally used to treat people who are infected with HIV, but research -- in gay and bisexual men and in straight couples with one infected partner -- have shown that it can lower the risk of infection in people who become exposed to the virus through sex. However, it does not eliminate the risk of infection.
The U.S. Food and Drug Administration approved the drug for prevention purposes in 2012.
Few people seem to be taking it for prevention purposes, however. Its manufacturer, Gilead, has disclosed that about 1,700 people are taking the drug for that reason in the United States, Grant said.
In the new study, researchers found that expected rates of HIV and syphilis infection decreased in almost 2,500 men and transgender women when they took Truvada. The study participants, who all faced high risk of HIV infection, were recruited in Peru, Ecuador, South Africa, Brazil, Thailand and the United States.
Some of the participants took Truvada while others took an inactive placebo. Those who believed they were taking Truvada "were just as safe as everyone else," Grant said, suggesting that they weren't more likely to stop using condoms or be more promiscuous because they believed they had extra protection against HIV infection.
Grant said the design of the study allows scientists to better understand the choices that participants make.
The study is limited, however, because the researchers recruited participants instead of waiting for people to come to them. For that reason, it's impossible to know if people will seek out Truvada to take new levels of risk by, say, no longer using condoms.
There are many skeptics, including the AIDS Healthcare Foundation, who fears that the drug will simply encourage people to make riskier decisions in regard to sex. One of these skeptics is Arleen Leibowitz, a professor emeritus of public policy at the Luskin School of Public Affairs at the University of California, Los Angeles.
She said the study shows that many people failed to take Truvada as prescribed and often didn't take enough to be protected from HIV. That raises the prospect that some people would take risks because they believe they're protected when they actually aren't, she noted.
Leibowitz also said some of the statistics in the study are questionable because they don't include enough participants. And she said the participants may have lied about their sex lives to please the people who interviewed them.
"We'll learn a lot when its use becomes more general," Leibowitz said. "But it's unfortunate to do experiments on the general population."
For the moment, she said, the drug may be appropriate for some patients who need protection from HIV, but doctors should be cautious and make sure their patients take the medication.
The study is published in the Dec. 18 online edition of the journal PLoS One.
In other HIV/AIDS news, a new study -- also published in PLoS One -- reports that 20-year-old men infected with HIV in the United States and Canada can expect to live almost as long as the general population and make it, typically, to their early 70s.
For more about HIV/AIDS, visit the U.S. National Library of Medicine.