Objective Since the recent introduction of preexposure prophylaxis (PrEP), several studies have reported a decrease in the use of condoms and a rise in STIs among users. This rise in risk behavior associated with the advent of PrEP is known as “risk compensation.” The aim of this study is to measure clinical and behavioral changes associated with the introduction of PrEP by analyzing condom use for anal intercourse, number of sexual partners, sexualized drug use and STI incidence. Methods We performed a retrospective descriptive study of PrEP users followed every 3months over a 2-year period spanning 2017–2019 in a referral clinic specializing in STI/HIV in Madrid, Spain. One hundred ten men who have sex with men and transgender women underwent regular screening for STIs and hepatitis C virus (HCV) infection. Sociodemographic, clinical, and behavioral data were gathered for all subjects studied. Results The risk compensation observed in this study consisted primarily of a lower rate of condom use, while the number of sexual partners and recreational drug consumption remained stable. We observed a very high incidence of STIs in this sample, particularly rectal gonorrhea and chlamydia. The factors shown to be independently associated with the presence of an STI on multivariate analysis were age below 30 years and over 10 sexual partners/month. Conclusion The incidence of STI acquisition was higher than expected, indicating a need for strategies to minimize this impact, particularly among younger individuals with a higher number of sexual partners.