When seat belt laws went into effect, some studies suggested drivers became more likely to speed or drive recklessly—a phenomenon called “risk compensation.” Is the same thing happening among people at risk of HIV, now that they can take antiretroviral drugs to protect themselves from the virus? A new study of condom use among gay and bisexual men in Australia suggests the answer is yes.

The study, published in The Lancet HIV today, surveyed nearly 17,000 men who have sex with men in Sydney and Melbourne between 2013 and 2017. Use of pre-exposure prophylaxis, or PrEP—daily antiretroviral pills taken by uninfected people—during the study years jumped from 2% of the HIV-negative participants to 24%. In the same time frame, “consistent” condom use dropped from 46% to 31% in men who reported having anal sex with casual partners. But just as seat belts can save lives even if risk compensation occurs, PrEP is so effective that it may more than offset the riskier behavior.

Some previous studies had found little change in condom use in people who use PrEP. Others, however, showed the introduction of PrEP has coincided with increases in sexually transmitted infections (STIs) like chlamydia and syphilis, a sign that condom use had dropped. But this, too, has been controversial because those increases could be due to increases in testing for STIs. The new study could help resolve the debate. It documents “an important trend,” says Kenneth Mayer, medical research director for lesbian, gay, bisexual, transgender, and questioning–oriented Fenway Institute in Boston.

What the study does not resolve is the impact of the risk compensation. The authors note that both the Sydney and Melbourne areas saw pronounced drops in HIV transmission rates between 2016 and 2017, coinciding with the increased use of PrEP. The study’s lead author, social scientist Martin Holt of the University of New South Wales in Sydney, says this likely occurred because the first public campaigns that advocated PrEP use targeted gay and bisexual men who reported inconsistent condom use with partners they did not know. “One theory is we’ve removed this massive group of guys who otherwise would be at high risk of HIV,” Holt says. “The initial results are promising, but I’m slightly concerned about the long-term effects.”

Holt says PrEP works so well that he is not much worried about the men who are using it. “There’s been a myopic focus on PrEP users and what do they do,” he says. Rather, he thinks more attention should be given to the men who are not on PrEP, whose behavior could be affected by the new sense of security among their peers. He says if condom use norms change and it “is not the default strategy used by gay men,” then HIV transmission rates could spike.

Mayer says “a more nuanced approach” is needed in both HIV and STI prevention efforts because the idea of “sexual health” is evolving now that the risk of condomless sex has dropped—both from PrEP and the fact that infected people on treatment rarely transmit the virus. He says patients who don’t want to use condoms all the time should be counseled to consider both PrEP and frequent STI screening.

Holt says the increasing use of PrEP around the world is essentially a “grand experiment” that is going to require adjustment in public health messaging campaigns. “The broader impact of PrEP on the community and their HIV protection needs is going to become a bigger and bigger issue.”