This report examines CMS's oversight of Medicare Part D beneficiaries who receive opioid prescriptions and providers who prescribe opioids to these beneficiaries. It finds that while CMS provides guidance to Part D plan sponsors on monitoring beneficiaries at high risk of opioid overuse, it lacks complete data on the full population of beneficiaries at risk. It also finds that CMS oversees prescribing through its contractor NBI MEDIC but does not specifically analyze opioid prescription data or require reporting on actions taken regarding inappropriate opioid prescribing. The report concludes that CMS needs more comprehensive oversight to reduce the risks of opioid misuse, overdose, and inappropriate prescribing among Medicare beneficiaries.