BackgroundThe emergence of chemsex has raised several concerns about gay, bisexual, and other men who have sex with men's (GBMSM) health. In this study we aim to analyze illicit drugs and chemsex use, sexual behavior and sexually transmitted infections (STI) in GBMSM who attended to a sexual health clinic and to explore any potential association between drug use and STI. MethodsWe conducted an observational study between January and June 2019 among GBMSM population attending to a STI clinic in Barcelona, Spain. An anonymous self-administered questionnaire was given consecutively to all participants older than 18 years who accepted to participate. ResultsA total of 514 GBMSM (median age of 34 years-old) were included. The median number of sexual partners in the last year was 20. Seventy-one percent did not use condoms consistently for receptive anal intercourse. Drug abuse prevalence in the preceding year was 64.2%, and 26.5% of the individuals practiced chemsex. Gamma-hydroxibutyrate/gammabutyrolactone, poppers and methamphetamine were the most common drugs in chemsex. Chemsex was associated to group sex (OR 9.8 [95 CI: 4–24]), HIV infection (OR 2.5 [95 CI: 1.1–5.8]), taking pre-exposure prophylaxis (OR 3.2 [95 CI: 1.5–7.1]), developing gonorrhea (OR 3.7 [95 CI: 1.5–8.8]) or syphilis (OR 6.7 [95 CI: 2.4–18.7]). ConclusionsThe prevalence of drug use and chemsex was high among GBMSM in Barcelona. Chemsex was associated with group sex, taking PrEP, and contracting syphilis, gonorrhea, and HIV.