PURPOSE: Nation-wide rapid declines in prescription opioid dispensing gave rise to concerns regarding restricted access to effective pain management for patients with cancer-related pain. One important mechanism for such restrictions could be through more restrictive insurance coverage for opioids. This study aims to assess recent changes in Medicare Part D formulary designs for opioids commonly used for cancer-related pain. METHODS: We used data from the 2015-2021 Medicare Prescription Drug Plan (PDP) Formulary Files to assess formulary changes for six opioid-dose combinations commonly used for cancer-related pain. We estimated % of PDPs adopting prior authorization, quantity limits (and limits adopted), and a higher cost-sharing tier for each opioid-dose combination. We further estimated median and mean out-of-pocket (OOP) costs across all PDPs for a 30-day supply of the drug. Trends in proportions were tested using the Cochrane-Armitage test; trends in continuous measures were tested using the Jonckheere-Terpstra test. RESULTS: Proportion of PDPs adopting prior authorization increased from close to 0% to about 50% for two long-acting opioids ( P < .001). Distribution of quantity limits across PDPs shifted over time to being more restrictive for all opioids considered ( P < .001). For four of the six opioids, the proportion of PDPs adopting tier 3 or above increased from below or about 50% to well over 70% ( P < .001). For the same four opioids, median OOP costs doubled to quadrupled ( P < .001). CONCLUSION: Medicare PDP coverage has become increasingly restrictive for opioids commonly used for cancer-related pain, with multifold increases in patient OOP costs over the past 7 years. These changes pose concerns for patients with cancer needing opioid therapies for pain control and call for strategies to effectively exempt cancer-related pain from insurance and pharmacy rules intended to apply to opioids for noncancer chronic pain.