BACKGROUND AND AIMS: Benzodiazepines (BZDs) carry a risk for drug overdose and are prescribed alone or simultaneously with selective-serotonin reuptake inhibitors (SSRIs) for the treatment of anxiety and depression in young adults. We aimed to measure risks of drug overdose following BZD treatment initiation, and simultaneous BZD and SSRI initiation, compared with SSRI treatment alone in young adults with depression or anxiety. DESIGN, SETTING, PARTICIPANTS: The cohort study used administrative databases covering privately (MarketScan, 1/1/2009-12/31/2018) and publicly (Medicaid, 1/1/2015-12/31/2016) insured young adults (18-29 years) in the United States. Those with depression or anxiety diagnoses newly initiating BZD or SSRI treatment (without BZD or SSRI prescriptions in prior year) were included. Simultaneous "BZD + SSRI" initiation was defined as starting BZD and SSRI treatment on the same day. The cohorts included 604 664 privately insured young adults (BZD = 22%, BZD + SSRI = 10%, SSRI = 68%) and 110 493 publicly insured young adults (BZD = 23%, BZD + SSRI = 5%, SSRI = 72%). MEASUREMENTS: Incident medically treated drug overdose events were identified from emergency department and inpatient encounters (ICD poisoning codes) within 6 months of treatment initiation. Crude and propensity-score adjusted cumulative incidence and hazard ratios (HR) were estimated. Sub-analyses evaluated drug overdose intent. FINDINGS: Adjusted HRs of drug overdose for BZD vs. SSRI treatment was 1.36 (95% confidence interval [CI]:1.23-1.51) in privately and 1.59 (95%CI:1.37-1.83) in publicly insured young adults. The adjusted HRs of drug overdose for BZD + SSRI treatment vs. SSRI treatment were 1.99 (95%CI:1.77-2.25) in privately and 1.98 (95%CI:1.47-2.68) in publicly insured young adults. CONCLUSIONS: Among young adults in the United States, initiating benzodiazepine treatment for anxiety and depression, alone or simultaneously with selective-serotonin reuptake inhibitors (SSRI), appears to have an increased risk of medically treated drug overdose compared with SSRI treatment alone. These associations were observed in publicly and privately insured individuals.