Imagine a daily pill that prevents an unwanted consequence of sexual intercourse. Does it give users a “license for promiscuity”? Will its widespread availability lead to “sexual anarchy”? These questions were posed more than a half-century ago about oral contraceptive pills, which enabled condomless heterosexual sex with far lower risk of pregnancy. 1

Similar concerns have now arisen about preexposure prophylaxis (PrEP) for HIV, especially when it’s prescribed to gay men and other men who have sex with men. PrEP, as currently approved by the US Food and Drug Administration (FDA), is a once-daily antiretroviral pill that is more than 90% effective in preventing HIV infection when taken as prescribed. But as with oral contraception, some people view PrEP as a double-edged sword: PrEP may protect people against acquiring HIV, but absent that risk, users might have more partners or more condomless sex, thereby increasing their risk of non-HIV sexually transmitted infections (STIs). This anticipated pattern of behavior—greater risk taking in response to an increased sense of protection—is in keeping with a theory called risk compensation. Clinicians’ concerns about risk compensation may be one reason for the slow uptake of PrEP in the United States.