Drs. Katz and Gerberding [1] recommend offering patients postexposure prophylaxis with antiretroviral drugs to prevent HIV infection after risky sexual activities. As they note, the availability of this prophylaxis could have profound behavioral, social, and economic consequences. In particular, the economic impact of postexposure prophylaxis could be substantial. The cost of a 4-week course of postexposure prophylaxis, including laboratory work and clinic visits, is approximately $1100 [2]. These costs must, of course, be balanced against the tremendous costs associated with failing to prevent HIV infection. But they must also be balanced against the costs of alternative strategies for preventing the spread of HIV.

Recent analyses suggest that postexposure prophylaxis could be a cost-effective strategy for preventing HIV infection in some circumstances, but not others [2-4]. The cost-effectiveness depends on the level of risk associated with the particular sex act and the likelihood that the sex partner is HIV-infected. Postexposure prophylaxis is generally cost-effective after receptive anal intercourse but not after receptive vaginal intercourse. An exception to the latter may be vaginal intercourse with someone known to be infected (postexposure prophylaxis after insertive intercourse is not cost-effective under any circumstances) [4].

Providing postexposure prophylaxis could cost tens of thousands to millions of dollars per HIV infection averted [4]. In contrast, many HIV risk-reduction interventions that encourage the adoption and maintenance of safer sex behaviors are actually cost saving (that is, they cost less than they save by averting HIV-related medical care costs) [5]. Indeed, for the $1100 that postexposure prophylaxis costs, an at-risk person could receive a comprehensive array of HIV prevention services, including 1) HIV counseling and testing; 2) participation in an intensive, multisession intervention based on sound principles of behavioral science; 3) booster sessions of the small-group intervention; and 4) a 1-year supply of condoms and sterile syringes. Thus, compared with other strategies for HIV prevention, postexposure prophylaxis may not represent an especially good "buy."