Lead author Jonathan Volk, a physician and epidemiologist at Kaiser Permanente San Francisco Medical Center, described the study as "the first to extend the understanding of the use of PrEP in a real-world setting and suggests that the treatment may prevent new HIV infections even in a high-risk setting."

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The Centers for Disease Control and Prevention says that PrEP has been shown to reduce the risk of HIV infection by up to 92 percent when taken consistently but is much less effective when taken inconsistently. In one key study, called PROUD that included men who have sex with men in Britain, the risk was reduced by 86 percent.

In this study, 100 percent of the participants remained HIV-free. That's right, not a single person in the study, published in Clinical Infectious Diseases, became infected while on the drug during the study period that included 2.5 years of observation.

"Tremendously good news," University of California-San Francisco researchers Kimberly A. Koester and Robert M. Grant (one of Time's most influential people of 2012 for his work in AIDS) said of the results in a commentary accompanying the publication of the study.

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Not long after the Food and Drug Administration first approved the drug for preventive HIV use in 2012, the Los Angeles-based AIDS Healthcare Foundation derided it as a "party drug" and warned that high-risk individuals would use it instead of condoms -- raising the risk of transmission of other sexually transmitted diseases. #Truvadawhore went viral. But as more studies have come out showing how well it appears to protect against HIV, many of those critics appear to be turning around.

Koester and Grant emphasized that despite the promising findings in the Kaiser study many questions still remain, a number of them practical in nature.

"What proportion of the population vulnerable to HIV will take a pill a day to prevent it? How will costs of the medication and clinic visits be paid for?" they asked. "Assuming people are willing to use PrEP and can access PrEP, will they take the medication as directed? Will uptake and use be higher or lower among those at higher risk? Will people place themselves at higher risk or HIV and sexually transmitted infections (STIs) as a consequence of using PrEP?"

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The pair said it wasn't clear from the study if the reported rate of sexually transmitted infections in the study is an increase or not and that further investigation is needed. They recommended that Truvada be combined with a parallel plan to prevent other STIs which may include the use of condoms, more frequent testing and discussions with prospective partners.

There are several dozen other ongoing clinical trials worldwide using Truvada in different populations. In South Africa, for instance, National Institutes of Health-funded researchers are looking at its use in heterosexual adolescent men and women ages 15 to 19 and in Australia the government is looking at people in relationships with HIV-negative partners. The issues they are hoping to find out more about include the factors that can influence an individual's compliance with taking the pills regularly, how effective the pill can be when it's not taken regularly but before and after sex, and how to integrate education about Truvada into regular clinic services.

Correction: A previous version of this story stated that another study on Truvada found that the drug prevented infection in about 86 percent of participants. The study, called PROUD, lowered their risk by 86 percent.

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This post has been updated.