Unintended Consequences of Life-Saving Pharmaceutical Innovations: How HAART Led to the Resurgence of Syphilis.

Syphilis, a sexually transmitted infection that can lead to serious health complications, was almost eliminated in the United States by 2000. But since then, its incidence began to increase, recently reaching a 60-year peak. We suggest that the introduction of the highly active antiretroviral therapy (HAART) drug regimen, which transformed HIV from a terminal condition into a manageable chronic disease, is partly responsible, as people living with and without HIV altered their sexual behavior after the introduction of HAART. To test this empirically, we exploit variation in HAART takeup based on spatial variation in pre-HAART AIDS prevalence, sex, and time in a triple differences framework. We find that a one standard deviation increase in the pre-HAART AIDS prevalence rate led to a 17.8 percent increase in the syphilis incidence rate, and that in the absence of HAART, there would have been 81 percent fewer syphilis cases between 1996 and 2008. These results highlight the need to consider unintended consequences that could stem from behavioral changes following the introduction of life-saving medical innovations.