Taking a pill as a preventive measure against HIV infection may not encourage people at high risk for the disease to engage in risky sexual behavior, according to a new U.S. study meant to address fears about its use.

The research, published in the Public Library of Science journal PLOS ONE, builds on the 2010 landmark study that found Gilead Sciences Inc’s Truvada - a pill already used widely to treat the human immunodeficiency virus - was more than 90 percent effective at preventing HIV infections among test subjects who took the drug as prescribed.

It is the latest to look at whether taking a pill to prevent infections could lead to “risk compensation,” or an adjustment to their behavior in response to their perceived level of risk.

“There has been a concern that really anything we do to prevent HIV in high-risk individuals could lead people to feel more secure and have riskier sex,” said Dr. Robert Grant of the Gladstone Institute of Virology and Immunology in San Francisco, one of the study’s authors.

Grant’s lab went back to its landmark iPrEx trial, a three-year study involving nearly 2,500 HIV‐negative gay men and male-to-female transgendered women in six countries.

The team looked specifically at a population of patients in the study who believed the treatment was working to see if risky behavior increased. They found rates of new HIV infections fell by 400 percent. Syphilis rates, a common sexually transmitted infection, also dropped.

Grant said there was no increase in sexual risk behavior, and the study even showed a trend toward safer sexual practices.

The findings were similar in a study published in October in the Lancet Infectious Diseases, which looked at Truvada in heterosexual couples where one person is infected and the other is not. In this study, however, there was a slight uptick in unprotected sex with outside partners.

Dr. Myron Cohen, director of the Institute for Global Health and Infectious Diseases at the University of North Carolina at Chapel Hill, said the trials aren’t likely to prove what might happen if the drug were to be used in a much larger population that was not under such close scrutiny.

“In general, it remains an open question about if you rolled out PrEP to a very large number of people what the long-range public health implications would be,” he said.

Truvada is one of several strategies called pre-exposure prophylaxis or PrEP, an experimental approach to HIV prevention where HIV negative people take HIV drugs to try to prevent infections. Besides Truvada, other PrEP methods being studied include long-acting injections, gels and vaginal rings.

According to the U.S. Centers for Disease Control and Prevention, as many as 500,000 high-risk individuals could benefit from the drug, but uptake has been slow.

A Gilead survey of Truvada use presented in September showed that only about 2,000 uninfected people in the United States are taking the drug to prevent HIV infection.

Nearly 1.2 million people in the United States live with HIV, and new infections are estimated at 50,000 each year.

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By Julie Steenhuysen