BACKGROUNDWe assess adverse events (AEs) following medication initiation for adolescents and young adults with opioid use disorder (OUD).METHODSThis is a secondary analysis of a clinical trial of long-acting injectable naltrexone (LAI-naltrexone) among youth with OUD aged 15 to 21 years. Participants were recruited from residential treatment and placed into 1 of 3 treatment groups based on medication receipt at time of discharge (no medication, sublingual buprenorphine-naloxone [buprenorphine], or LAI-naltrexone). Frequencies and percentages of AEs by body system were compared by medication group at the 1-month follow-up visit. Logistic regression was used to compare groups on their likelihood of reporting an AE, overall and excluding injection site reactions.RESULTSOf 199 participants, 71 (36%) received no medication, 59 (30%) buprenorphine, and 69 (35%) LAI-naltrexone at discharge. Participants who received LAI-naltrexone experienced more AEs, primarily due to injection site reactions (62%, accounting for 43% of all AEs among participants who received LAI-naltrexone). There were 6 reports of nonlethal overdose, 5 in the no medication, 1 in the buprenorphine, and none in the LAI-naltrexone group. Participants receiving LAI-naltrexone were more likely to report an AE compared to the other groups (P = .04), but this difference was no longer significant when excluding injection site reactions (P = .82).CONCLUSIONSExcluding injection site reactions, there were no significant differences in the likelihood of reporting an AE 1 month after receiving LAI-NTX, buprenorphine, and no medications. LAI-naltrexone should be among the medications offered for the treatment of OUD in youth.