Chronic opioid use for noncancer pain is a well-established patient safety problem but less is known about chronic use after cancer treatment. This study used ten years of national Medicare surveillance data to assess the risks of serious adverse events associated with continued prescription opioid use after treatment for breast cancer. Findings indicate that opioid use after active cancer treatment was associated with a 15-fold increase in risk of adverse events (including overdose) and an increased risk in adverse events related to opioid use, including falls, fractures, and hospitalization. These findings suggest that practitioners may be able to improve outcomes by closely monitoring and reducing unnecessary long-term opioid use after cancer treatment.