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A little blue pill called Truvada could effectively prevent people from ever getting HIV — but only if they use it perfectly.

A study published this week in Clinical Infectious Diseases followed 657 patients who took Truvada, a prophylactic that protects against HIV, for almost three years. The participants were HIV-negative, but at high risk of contracting the virus. Yet during the course of the study, none of them did.

That’s led some publications to hail the drug as “100% effective.” It’s important to note, however, that this new study was not a clinical trial; there was not a control group that did not take the drug, so it’s impossible to say how many more cases there would have been in a non-Truvada group. Still, the results were encouraging.

Back in 2010, the clinical trial that led to the drug’s 2012 approval found that Truvada was 99% effective at protecting gay men from the virus. (It had already been approved years earlier as a treatment for people who were already HIV positive.)  According to the CDC, it’s “been shown to reduce the risk of HIV infection in people who are at high risk by up to 92%.”

But there’s a catch. …
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See a follow-up story, too:

• There’s an almost-perfect HIV prevention pill, but not enough people are using it