BACKGROUND:Pre-exposure prophylaxis (PrEP) is an effective method for reducing HIV incidence among at-risk populations. However, concerns exist over the potential for an increase in STIs following PrEP initiation. The objective of this study is to compare the STI incidence before and after PrEP initiation within subjects among a cohort of men who have sex with men in Los Angeles, California. METHODS:The present study used data from patients who initiated PrEP services at the Los Angeles LGBT Center between October 2015 and October 2016 (n=275). A generalised linear mixed model was used with a case-crossover design to determine if there was a significant difference in STIs within subjects 365 days before (before-PrEP period) and 365 days after PrEP initiation (after-PrEP period). RESULTS:In a generalised linear mixed model, there were no significant differences in urethral gonorrhoea (P=0.95), rectal gonorrhoea (P=0.33), pharyngeal gonorrhoea (P=0.65) or urethral chlamydia (P=0.71) between periods. There were modest increases in rectal chlamydia (rate ratio (RR) 1.83; 95% CI 1.13 to 2.98; P=0.01) and syphilis diagnoses (RR 2.97; 95% CI 1.23 to 7.18; P=0.02). CONCLUSIONS:There were significant increases in rectal chlamydia and syphilis diagnoses when comparing the periods directly before and after PrEP initiation. However, only 28% of individuals had an increase in STIs between periods. Although risk compensation appears to be present for a segment of PrEP users, the majority of individuals either maintain or decrease their sexual risk following PrEP initiation.