ObjectiveTo study the comparative effectiveness of angiotensin receptor blockers (ARB) and angiotensin-converting enzyme inhibitors (ACEi) in ethnic minority groups in the UK.DesignObservational cohort study using a reference trial emulation approach benchmarked against the ONTARGET trial.SettingUK Clinical Practice Research Datalink Aurum data from 01/01/2001-31/07/2019. Participants Black, South Asian, or White patients with a prescription for an ARB/ACEi who met the ONTARGET trial criteria.Main outcome measuresThe primary composite outcome was: cardiovascular-related death, myocardial infarction, stroke, or hospitalisation for heart failure with individual components studied as secondary outcomes. Angioedema was a safety endpoint. We assessed outcomes using a propensity-score-weighted Cox proportional hazards model for ARB vs ACEi with heterogeneity by ethnicity assessed on the relative and absolute scale.Results17,593 Black, 30,805 South Asian, and 524,623 White patients were included. We benchmarked results against ONTARGET comparing ARB with ACEi for the primary outcome (hazard ratio [HR] 0.96, 95% CI: 0.95 to 0.98) and found no evidence of treatment effect heterogeneity(Pint=0.422). Results were consistent for most secondary outcomes. However, for cardiovascular-related death, there was strong evidence of heterogeneity (Pint=0.002), with ARB associated with more events in Black individuals and with fewer events in White individuals compared to ACEi, and no differences in South Asian individuals. For angioedema, HR 0.56 (95% CI: 0.46 to 0.67) for ARB vs ACEi (Pint=0.306). Absolute risks were higher in Black individuals, with number-needed-to-harm of 204 in Black individuals compared with 2000 in South Asian individuals and 1667 in White individuals (Pint=0.023).ConclusionsThese results demonstrate variation in drug effects of ACEi and ARB by ethnicity and suggest the potential for adverse consequences from current UK guideline recommendations for ARB in preference to ACEi for Black individuals.