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Existing studies cannot evaluate the time-varying properties of frailty and polypharmacy despite raising concerns about their combined effects in older individuals. This study investigated the longitudinal association between different combined statuses of frailty and polypharmacy on risks of adverse outcomes. Retrospective cohort study. Subjects aged between 65 and 100 years (n = 100,000) were identified from Taiwan's National Health Insurance Research Database. Frailty was categorized into fit, mild, moderate, and severe frailty based on the multimorbidity frailty index. Concomitant use of 5 to 9 and ≥10 chronic medications was considered polypharmacy and excessive polypharmacy. We used generalized estimating equation models to examine the association among 12 groups of combined effects of frailty and polypharmacy and risks of all-cause mortality, all-cause hospitalization, and unplanned hospitalization. Age-stratified analyses were conducted for those aged 65 to 74, 75 to 84, and 85+ years. Compared with fit without polypharmacy, severe frailty with excess polypharmacy was associated with increased risks of adverse outcomes, particularly unplanned hospitalization (adjusted relative risk (aRR): 20.01 [95% confidence interval (CI)] 19.30–20.75). However, the combined effects varied in distinct groups. Within each polypharmacy category, there were dose-response associations between frailty category and adverse outcomes. For instance, within the polypharmacy group, the aRRs of mortality were 1.58 (1.52–1.64), 2.70 (2.60–2.80), 4.62 (4.44–4.82), and 6.81 (6.50–7.13) for the fit and mild, moderate, and severe frailty groups, respectively. By contrast, within each frailty category, the dose-response association between polypharmacy and adverse outcomes was limited to fit and mildly frail people. Age-stratified analyses yielded similar results. Both frailty and polypharmacy modified the risks of mortality and hospital admissions in older people, but the combined effects varied in distinct groups. This study thus highlights the potential to optimize care of older people by capturing the dynamic and combined changes related to frailty and polypharmacy. [ABSTRACT FROM AUTHOR]