BackgroundSubstance use is highly prevalent among men who have sex with men (MSM) and is associated with individual-level sexual risk behaviors. However, few studies have explored the relationship between substance use and HIV risk behaviors within partnerships. MethodsWe examined partner-level data between MSM participants (n=23) and their sexual partners (n=52). We used multivariable generalized estimating equations (GEE) logistic regression to assess the relationship between partner-level substance use during their last sexual encounter with each partner, and engaging in condomless anal intercourse (CAI) and serodiscordant CAI. ResultsIn multivariable analyses, participants had significantly higher adjusted odds ratio (AOR) of CAI when the participant (AOR=22.2, 95%CI=2.5–199.5) or their partners used any drugs (AOR=21.8, 95%CI=3.3–144.3); their partners (AOR=5.7, 95%CI=1.7–19.3) or both participant and partner had concordant use of methamphetamine (AOR=10.5, 95%CI=2.2–50.6); or when both used poppers (AOR=11.4, 95%CI=1.5–87). There were higher odds of SDCAI if the participant binge drank (AOR=4, 95%CI=1.01–15.8), used more than one substance (AOR=15.8, 95%CI=1.9–133), or used other drugs (AOR=4.8, 95%CI=1.3–18.4); if their partner used poppers (AOR=7.6, 95%CI=1.5–37.6), or used more than one substance (AOR=7.9, 95%CI=1.9–34.1); and when both participant and partner had concordant use of poppers (AOR=4.4, 95%CI=1.2–16.8). ConclusionsThis study observed significant relationship between substance use and HIV risk behaviors within partnerships. Specifically, when either the participant, the partner, or both used any drugs there was an increased odds of sexual risk behaviors. Findings suggest that partner-level substance use behaviors should be taken in account when developing sexual risk reduction interventions.