The CDC announced some deeply disconcerting news: Sexually transmitted infection rates have skyrocketed over the last year, particularly among gay and bisexual men. Syphilis transmission rates are especially alarming: The CDC reports a 15.1% increase in new infections nationwide since 2013, with a stunning 83% of male cases affecting men who have sex with men.

What’s going on here? One obvious explanation could be the rise of PrEP (or Truvada), a daily pill, which, all available data indicate, fully protects against HIV infection. Early studies of PrEP showed no evidence that those who took the drug would stop using condoms. But recent surveys suggest that at least some PrEP users do indeed use condoms less regularly once they’re on the pill. Most notably, a fairly small-scale study in San Francisco found a 30% instance in STIs among PrEP users after six months—which rose to a 50% after one year.* Just as troublingly, 41% of subjects admitted to using condoms less frequently while on PrEP.

The San Francisco study was too small to prove that PrEP is closely correlated to decreased condom use. But combined with the new CDC report, its findings should set off alarm bells. Is San Francisco our first preview of a dangerous new PrEP culture, where many gay and bisexual men ignore the risks of STIs once a critical mass of men are on PrEP? That would be the worst-case scenario, and there’s no reason to assume it’s true. But the possibility is worth considering—and, if feasible, forestalling. Read more via Slate