Pre-exposure prophylaxis (PrEP) is a new preventive treatment for individuals at high-risk for HIV infection, such as men who has sex with men (MSM). Studies have confirmed the efficacy but concerns about the potential induction of risk compensation remains. We aimed to assess the incidence of sexually transmitted infections (STIs) after PrEP initiation as a proxy for sexual risk behavior.This case-crossover study used data from medical records and from the Danish Microbiology Database from patients who initiated PrEP at the Region of Southern Denmark between 2017 and 2019. Poisson regression was used to assess STI incidence 6 months after PrEP initiation versus the 6 months before. To identify potential risk factors, we compared individuals with an increased STI incidence after PrEP initiation with those without, using logistic regression.In total, 46 MSM initiated PrEP in the study period. We found a significant increase in the number of positive samples for STI after PrEP initiation (IRR 1.83; 95% CI [1.03, 3.26]) and a tendency for higher incidence of STI episodes (1.67; 95% CI [0.91, 3.13]). The increase was concentrated to a group of users, but no significant correlation was found between increasing incidence and the baseline factors examined.We observed a degree of risk compensation after the implementation of PrEP among MSM, clustering to a group of users. Our results highlight the importance of frequent STI screening among MSM on PrEP as timely diagnosis could contribute to an overall decrease in STI incidence and incidence among MSM.
