Methadone and buprenorphine are commonly prescribed to treat opioid use disorder, but their use presents patient safety risks. Using national data from England and Wales, this study analyzed 2,284 patient safety incident reports and found that harmful incidents involving opioid substitution treatment with methadone or buprenorphine in community-based care stemmed from errors in dispensing practices (e.g. wrong patient, incorrect dose, incorrect formulation). Staff- and organization-related factors – such as not following protocols, poor continuity of care – contributed to more than half of the incidents.