Dose-response associations between number and frequency of substance use and high-risk sexual behaviors among HIV-negative substance-using men who have sex with men (SUMSM) in San Francisco.

We evaluated the relationship between frequency and number of substances used and HIV risk [ie, serodiscordant unprotected anal intercourse (SDUAI)] among 3173 HIV-negative substance-using MSM. Compared with nonusers, the adjusted odds ratio (AOR) for SDUAI among episodic and at least weekly users, respectively, was 3.31 [95% confidence interval (CI), 2.55 to 4.28] and 5.46 (95% CI, 3.80 to 7.84) for methamphetamine, 1.86 (95% CI, 1.51 to 2.29) and 3.13 (95% CI, 2.12 to 4.63) for cocaine, and 2.08 (95% CI, 1.68 to 2.56) and 2.54 (95% CI, 1.85 to 3.48) for poppers. Heavy alcohol drinkers reported more SDUAI than moderate drinkers [AOR, 1.90 (95% CI, 1.43 to 2.51)]. Compared with nonusers, AORs for using 1, 2, and ≥3 substances were 16.81 (95% CI, 12.25 to 23.08), 27.31 (95% CI, 18.93 to 39.39), and 46.38 (95% CI, 30.65 to 70.19), respectively. High-risk sexual behaviors were strongly associated with frequency and number of substances used.