42 CFR §423.466

1. (a)Retroactive claims adjustments, underpayment refunds, and overpayment recoveries. Whenever a sponsor receives information that necessitates a retroactive claims adjustment, the sponsor must process the adjustment and issue refunds or recovery notices within 45 days of the sponsor's receipt of complete information regarding claims adjustment.
2. (b)Coordination of benefits. Part D sponsors must coordinate benefits with SPAPs, other entities providing prescription drug coverage, beneficiaries, and others paying on the beneficiaries' behalf for a period of 3 years from the date on which the prescription for a covered Part D drug was filled.