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OBJECTIVE: To assess antiretroviral therapy (ART) adherence among people with HIV (PWH) in Canada and identify baseline characteristics associated with suboptimal adherence (<95%). DESIGN: Retrospective observational study using data from the National Prescription Drug Utilization Information System and Régie de l'assurance maladie Quebec (RAMQ) Public Prescription Drug Insurance Plan. METHODS: This analysis included PWH aged 18 years or older who initiated an ART regimen and were followed for at least 12 months (2010-2020). Patient characteristics were summarized using medical/pharmacy claims data from seven provinces (Alberta, Manitoba, New Brunswick, Newfoundland and Labrador, Ontario, Saskatchewan, and Quebec). ART regimen at index date (first dispensing of a regimen including a core agent) was defined as a single-tablet or multitablet regimen (MTR). Adherence was calculated using a Proportion of Days Covered approach, based on ART dispensing, recorded between April 2010 and the last available date. Multivariate linear regression analysis was used to determine correlations between suboptimal adherence and baseline characteristics. RESULTS: We identified 19 322 eligible PWH, 44.7% of whom had suboptimal adherence (<95%). Among 12 594 PWH with evaluable baseline data, 10 673 (84.8%) were ART-naive, 74.2% were men, mean age was 42.9 years, and 54.1% received a MTR as their ART. Based on multivariate regression analysis, suboptimal adherence was significantly associated with multitablet ART ( P  < 0.001) and younger age ( P  < 0.001) but not sex. CONCLUSION: Almost half of adult PWH in Canada had suboptimal adherence to ART. Better understanding of factors influencing adherence may help address gaps in current care practices that may impact adherence.