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Objective: The Z-drugs (zolpidem, zopiclone, zaleplon) are widely used to treat insomnia in patients receiving prescription opioids, and the risk of overdose resulting from this coprescription has not been explored. The authors compared the rates of overdose among patients using opioids plus Z-drugs and patients using opioids alone., Methods: All individuals 15 to 85 years of age receiving prescription opioids, regardless of underlying indication and without evidence of cancer, were identified in the IBM MarketScan database (2004-2017). Patients with concomitant exposure to Z-drugs were matched 1:1 to patients with exposure to prescription opioids alone based on opioid prescribed, morphine equivalents, number of days' supply, and hospitalization within the past 30 days. The primary outcome was any hospitalization or emergency department visit due to an overdose within 30 days, using an intention-to-treat approach. Fine stratification on the propensity score was used to control for confounding., Results: A total of 510,529 exposed patients and an equal number of matched reference patients were analyzed. There were 217 overdose events among the exposed patients (52.5 events per 10,000 person-years) and 57 events among the reference patients (14.4 events per 10,000 person-years), corresponding to an unadjusted hazard ratio of 3.67 (95% CI=2.75, 4.90). Using fine stratification on the propensity score (c-statistic: 0.66), the adjusted hazard ratio was 2.29 (95% CI=1.79, 2.91). Results were consistent across sensitivity analyses., Conclusions: Among patients receiving prescription opioids, after controlling for all confounding factors, concomitant treatment with Z-drugs was associated with a substantial relative increase in the risk of overdose. The potential implications are significant given the large number of opioid-treated patients receiving Z-drugs.