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Background: Gait speed decline, an early marker of functional impairment, is a sensitive predictor of adverse health outcomes in older adults. The effect of potentially inappropriate medications, including drug-disease and drug-drug interactions, on gait speed decline is not well known. Objective: The aim of this study was to determine if drug interactions impair functional status as measured by gait speed. Methods: The sample included 2402 older adults with medication and gait speed data from the Health, Aging and Body Composition study. The independent variable was the frequency of drug-disease and/or drug-drug interactions at baseline and 3 additional years. The main outcome was a clinically meaningful gait speed decline of ≥0.1 m/s the year following drug interaction assessment. Adjusted odds ratios and 95 % confidence intervals (CIs) were calculated using multivariate generalized estimating equations for both the overall sample and a sample stratified by gait speed at time of drug interaction assessment. Results: The prevalence of drug-disease and drug-drug interactions ranged from 7.6 to 9.3 and 10.5 to 12.3 %, respectively, with few participants (3.8-5.7 %) having multiple drug interactions. At least 22 % of participants had a gait speed decline of ≥0.1 m/s annually. Drug interactions were not significantly associated with gait speed decline overall or in the stratified sample of fast walkers. There was some evidence, however, that drug interactions increased the risk of gait speed decline among those participants with slower gait speeds, though p values did not reach statistical significance (adjusted odds ratio 1.22; 95 % CIs 0.96-1.56; p = 0.11). Moreover, a marginally significant dose-response relationship was seen with multiple drug interactions and gait speed decline (adjusted odds ratio 1.40; 95 % CIs 0.95-2.04; p = 0.08). Conclusions: Drug interactions may increase the likelihood of gait speed decline among older adults with evidence of preexisting debility. Future studies should focus on frail elders with less physiological reserve who may be more susceptible to the harms associated with potentially inappropriate medications. [ABSTRACT FROM AUTHOR]