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Background: In British Columbia, antiretrovirals (ARVs) for HIV treatment (HIV-Tx) and pre-exposure prophylaxis (PrEP) are free-of-charge through publicly-funded Drug Treatment Programs (DTPs). When available, less costly generics are substituted for brand-name ARVs. We describe the incidence and type of product substitution issue (PSI) adverse drug reactions (ADRs) attributed to generic ARVs., Methods: Cohorts included DTP clients ≥19 years who received generic ARVs for HIV-Tx (abacavir-lamivudine, emtricitabine-tenofovir DF, efavirenz-emtricitabine-tenofovir DF, atazanavir or darunavir between 01 Jun 2017 and 30 Jun 2022) or PrEP (emtricitabine-tenofovir DF, 01 Apr 2018 to 30 Jun 2022). Demographic, ARV and ADR data were extracted from DTP databases and summarized by descriptive statistics. PSI incidence was calculated for each product during the year following brand-to-generic and generic-to-generic transitions (first-year-post-rollout), and compared between generic versions using generalized estimating equations. For context, incidence of any ARV product-related ADR was calculated in the same 1-year periods., Results: During first-year-post-rollout periods, 5339 HIV-Tx (83% male, median age 52 years) and 8095 PrEP (99% male, median 33 years) clients received generic ARVs, and reported 78 and 23 generic PSIs, respectively. PSI incidence was <1% for most generic ARVs, with mild-moderate symptoms including gastrointestinal upset, headache, dizziness, fatigue/malaise and skin rash. In HIV-Tx clients, the efavirenz-containing product had higher PSI incidence than other ARVs (2.2%, p = .004), due to more neuropsychiatric adverse reactions. Any ADR incidence was stable across measurement periods, and generic PSIs represented less than one third of all product-related ADRs., Conclusions: Generic substitution of antiretrovirals for HIV-Tx and PrEP was well tolerated, with ≤2% incidence of mild-moderate PSI ADRs., Competing Interests: Declaration of conflicting interestsThe author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: JSGM’s Treatment as Prevention (TasP®) research, paid to his institution, has received support from the BC Ministry of Health, Health Canada, Canadian Institutes of Health Research, Public Health Agency of Canada, Genome Canada, Genome BC, Vancouver Coastal Health and VGH Foundation. Institutional grants have been provided by Gilead Sciences Inc, Janssen, Merck Sharp & Dohme LLC, and ViiV Healthcare. MH has received honoraria, administered by the institution, for consulting fees and advisory board participation from Gilead Sciences Canada, Merck Canada, and ViiV Healthcare. All other authors declare they have no conflicts of interest.