By Alice Park
November 19, 2015

Since 2012, when the FDA approved a drug that could prevent HIV transmission, critics have worried that the mere existence of such a pill would promote unsafe sex and cause HIV infections to surge. But a new study, published just before World AIDS Day, proves them wrong.

Reporting in JAMA Internal Medicine, researchers show that providing PrEP, short for pre-exposure prophylaxis, to men who are at high risk of contracting HIV dropped their rates of HIV infection dramatically. In the study, conducted at health centers in three cities, 437 men and transgender women took a PrEP drug called Truvada for nearly a year. In that time, all but two people remained HIV-free. (Those who were infected showed extremely low blood levels of the drug, indicating that they took only about half their required doses.)

Even more important, Truvada didn’t appear to make users more promiscuous or reckless about their risk. The people who reported engaging in the riskiest behaviors for getting infected also showed the highest blood levels of the drug, meaning they were taking their daily doses. In that time, the incidence of other sexually transmitted diseases (which Truvada doesn’t treat) remained high but also didn’t go up.

Earlier studies have shown that PrEP can lower a person’s risk of getting HIV by as much as 90%. But because the drug was tested in lab-based research settings, experts questioned whether it would work in the real world, where people are much less likely to dutifully take their pills at the proper doses.

Encouraging as the study results are, the existence of PrEP alone can’t stop HIV’s spread. “These studies show yet again that PrEP works,” says Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, which provided one of the grants to support the research. “The issue is, Can we get PrEP to the people who really need it?”

In their analysis, the researchers noticed a worrisome trend: while all the PrEP takers showed protective levels of the drug in their blood, they were lowest in African Americans, suggesting that those men were not taking the drug every day. Dr. Albert Liu, one of the authors from the San Francisco department of public health, believes that cultural barriers–perceptions about HIV and mistrust of the medical system–may be contributing to that lower adherence.

Fauci and Liu both note that if it’s not distributed in the right way, there’s a danger that PrEP will further entrench disparities in HIV incidence and promote resistance to the drug. PrEP was provided free to the study subjects but costs $8,000 to $14,000 a year. And while cities like San Francisco make PrEP available at no charge to anyone who is HIV-negative but at high risk, most cities have not allotted such funds for HIV prevention. “We know if we prevent an infection, it saves more than $350,000 over a lifetime for a person in health costs,” says Fauci. “So it’s definitely an economically sound approach.”

It’s time, he says, to follow San Francisco’s lead and figure out ways to make PrEP available to those who can benefit most. “Enough is enough,” Fauci says. “We have enough data, so let’s do it.”

Contact us at editors@time.com.

This appears in the November 30, 2015 issue of TIME.

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