Did life-saving HIV drugs unintentionally fuel a syphilis comeback?

By Dr. Sanchari Sinha Dutta, Ph.D.Reviewed by Lauren HardakerApr 27 2026

A life-saving HIV breakthrough reshaped survival. But did it also reshape sexual behavior and spark an unexpected public health challenge? 

Study: Unintended Consequences of Life-Saving Pharmaceutical Innovations: How HAART Led to the Resurgence of Syphilis. Image credit: Sonis Photography/Shutterstock.com

A recent study published in Health Economics reveals that the breakthrough discovery of antiretroviral therapy for HIV might have unintentionally triggered the resurgence of syphilis, a sexually transmitted infection, in the United States.

HIV survival gains coincide with shifting STI trends

The widespread use of Penicillin and changes in sexual norms have caused a more than 97 % drop in the incidence rate of syphilis, a sexually transmitted infection, in the United States between 1943 and 2000.

During this period, human immunodeficiency virus (HIV) emerged as a leading cause of death, and by 1993 was the most common cause of death among individuals aged 25–44, particularly among men aged 25–44 who have sex with men.

The discovery of highly active antiretroviral therapy (HAART) was a major medical breakthrough in 1996, which showed immense promise in effectively suppressing the replication of HIV over long periods. With the use of HAART, health professionals finally became successful in transforming HIV from a mostly fatal condition to a chronic, manageable one. The HIV-related mortality reached its all-time low value of 4748 deaths in 2023 due to the widespread use of HAART.

Unfortunately, the introduction of HAART coincided with a sudden reversal in the long-standing decline of syphilis incidence, which continued to increase sharply in the country, reaching a 60-year peak of 62.2 per 100,000 in 2022.

Considering this overlapped period of two major events, researchers from the University of Texas, Baylor University, and the University of North Carolina hypothesized that HAART-mediated transformation of HIV from a terminal condition into a manageable chronic disease is partly responsible for the observed resurgence in syphilis incidence.

Possible mechanisms driving syphilis resurgence

The researchers considered two mechanisms that may potentially contribute to the resurgence of syphilis in the United States.

The first mechanism might be an increase in high-risk sexual behavior among people living with or without HIV due to HAART-mediated improvement in health conditions, as well as changes in perceived HIV risk associated with treatment. However, a reduction in the perceived risk of viral transmission might not be highly relevant in this context, as there was a lack of widespread public messaging highlighting this reduced risk until the late 2000s.

The second mechanism might be an increased possibility of contracting sexually transmitted infections as a natural consequence of the increased longevity of HIV-infected people, owing to the introduction of HAART, which saved the lives of thousands of HIV-infected people and allowed them to lead relatively normal lives.

Evidence supporting these mechanisms

The researchers comprehensively analyzed information from the Centers for Disease Control and Prevention (CDC) and pharmaceutical industry sales data to provide evidence that both mechanisms likely contributed. However, increased longevity alone cannot fully explain the observed trends.

The analysis revealed that with the introduction of HAART, syphilis incidence started increasing differentially among men in states that previously exhibited higher prevalence of acquired Immunodeficiency Syndrome (AIDS), the most advanced stage of HIV infection. These states had the most widespread prescriptions of HAART and the highest reduction in HIV-related mortality.

Specifically, the analysis indicated that the introduction of HAART led to approximately 71,190 additional syphilis diagnoses between 1996 and 2008. In a counterfactual scenario without HAART, there would have been about 71,190 fewer cases over this period, representing an approximately 81 % reduction compared with observed cases. Notably, the incidence rate of syphilis among women continued to decline.

Further analysis revealed that the observed increase in syphilis incidence among men cannot be solely explained by increased longevity among people living with HIV, highlighting a significant role of high-risk sexual behavior inferred from incidence patterns in driving such an unintended consequence of a major medical breakthrough.

Furthermore, several analyses checking the robustness of the findings revealed that the observed rise in syphilis incidence is not driven by changes in drug use, changes in social attitudes toward men having sex with men, or variation in public health funding.

Findings highlight unintended public health trade-offs of innovation

The study highlights that behavioral shifts of people towards high-risk sexual behavior, such as unprotected sex, as well as increased longevity following the introduction of HAART, might have triggered the resurgence of syphilis by increasing the risk of contracting sexually transmitted infections.

Given the findings, the researchers suggest developing complementary interventions to address behavioral changes, unintentionally triggered by HAART, to mitigate some of its negative consequences. They suggest expanding public health campaigns to promote safer sexual practices and increasing screening for sexually transmitted infections.

They specifically emphasize that the study findings should not be interpreted as a criticism of HAART itself, since the benefits of life-saving HAART far outweigh the costs associated with the observed unintended consequences.

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