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BackgroundLittle is known regarding the degree to which substance and alcohol use effects on HIV viral suppression are mediated through medication adherence. We hypothesized that the total effects of such use are mediated through adherence.MethodsWe included patients with HIV (PWH) receiving care at an urban academic HIV clinic between 2014 and 2018. Eligible patients were those prescribed antiretroviral therapy who completed both patient reported outcome (PRO) questionnaires, and had subsequent plasma viral load (pVL) measurements. Measures included assessments of alcohol use (AUDIT-C), drug use (ASSIST), and self-reported adherence. Substances found in bivariate analysis to predict detectable pVL were modeled separately for mediation effects through adherence. We report natural direct (NDE) and indirect effect (NIE), marginal total effect (MTE) and percentage mediated.ResultsAmong 3125 Patients who met eligibility criteria, percentages of current use by category were: hazardous alcohol 25.8%, cannabis 27.1%, amphetamines 13.1%, inhalants 11.9%, cocaine 5.3%, sedative-hypnotics 4.5%, opioids 2.9%, and hallucinogens 2.3%. Excellent adherence was reported in 58% and 10% had detectable pVL. Except for sedatives use of other ascertained substances was significantly associated with worse adherence. Bivariate predictors of detectable pVL were [OR(95% CI)]: amphetamine use 2.4 (1.8 -3.2) and opioid use 2.3 (1.3 - 4.0). The percentage mediated by adherence was 36% for amphetamine use, 26.5% for opioid use, and 39% for multiple substance use.ConclusionUse of amphetamines, opioids, and multiple substances predicted detectable pVL. However, less than 40% of effects were mediated by self-reported adherence.SummaryWe examined adherence-mediated effects of hazardous alcohol and substance use on HIV viral suppression. Use of amphetamines, opioids, and multiple substance predicted detectable viral load, however, less than 40% of effects were mediated by self-reported antiretroviral adherence.