OBJECTIVES It is unclear how long youth with opioid use disorder (OUD) should continue taking buprenorphine, and what adherence they should achieve. We identified patterns of duration/adherence and assessed associations with subsequent overdose, emergency department (ED) use, and hospitalization. METHODS This retrospective cohort analysis used 2014–2022 data from the Massachusetts Public Health Data Warehouse. We identified youth aged 13 to 26 years initiating buprenorphine and used group-based trajectory modeling to categorize youth into duration/adherence trajectories over 12 months. Using multivariable Cox regression, we examined associations between trajectories and time to fatal/nonfatal opioid overdose, all-cause ED use, and all-cause hospitalization during the subsequent 12-month period. RESULTS Among 11 649 Massachusetts youth initiating buprenorphine, most were aged 21 years or older (89.0%), male (60.3%), white non-Hispanic (85.9%), and enrolled in Medicaid (55.4%). We identified 4 patterns of medication use: (1) high adherence for 12 months (23.7%); (2) low adherence for 12 months (27.5%); (3) discontinuation in 3 to 9 months (16.4%); and (4) discontinuation in less than 3 months (32.5%). Trajectories included 580 (5.0%) and 774 (6.6%) youth switching to methadone and naltrexone, respectively. Compared with high adherence for 12 months, overdose risk was higher with low adherence for 12 months (adjusted hazard ratio [aHR], 1.46; 95% CI, 1.24–1.73), discontinuation in 3 to 9 months (aHR, 1.82; 95% CI, 1.52–2.17), and discontinuation in less than 3 months (aHR, 1.76; 95% CI 1.50–2.06). Compared with high adherence, low adherence and discontinuation in less than 3 months had higher risk of ED use, and all other trajectories had higher risk of hospitalization. CONCLUSIONS Medication adherence may prevent overdose, ED use, and hospitalization. Strategies to increase treatment duration/adherence likely avert harm.