On September 28, 2010, CMS posted a letter to State Medicaid Directors on implementation of the ACA Medicaid drug rebate provisions. This guidance revises the previous policy concerning the federal offset of Medicaid prescription drug rebates, and further specifies the process CMS will use for the estimation and collection of these offsets. It also provides information on rebates for Medicaid managed care organization (MCO) drugs, MCO formularies, and the treatment of MCO physician-administered drugs. In addition, the guidance addresses manufacturer reporting requirements, the treatment of discounts under the Medicare Coverage Gap Discount Program for purposes of determining best price, and changes to Medicaid excluded drug provisions.