The GAO has sent a letter to Congress describing how CMS monitors Medicare Part D prescription drug plan sponsors' compliance with mid-year formulary change requirements. CMS oversight activities include reviews of requests prior to implementation; discussions with trade groups, advocates, and other patient representatives; and targeted audit activities. Based on a review of mid-year formulary changes sponsors made in 2008 and 2009, the GAO found that most of these changes (87.4% and 88.6%, respectively) were formulary enhancements that added a drug to the formulary or removed/loosened restrictions. In 2008, about 5% of beneficiaries filled a prescription for a drug that was later affected by a negative mid-year change.