42 CFR §423.124

(a) Out-of-network access to covered part D drugs—(1) Out-of-network pharmacy access. A Part D sponsor must ensure that Part D enrollees have adequate access to covered Part D drugs dispensed at out-of-network pharmacies when the enrollees—

(i) Cannot reasonably be expected to obtain such drugs at a network pharmacy; and (ii) Do not access covered Part D drugs at an out-of-network pharmacy on a routine basis. (2) Physician's office access. A Part D sponsor must ensure that Part D enrollees have adequate access to vaccines and other covered Part D drugs appropriately dispensed and administered by a physician in a physician's office. (b) Financial responsibility for out-of-network access to covered Part D drugs. A Part D sponsor that provides its Part D enrollees with coverage other than defined standard coverage may require its Part D enrollees accessing covered Part D drugs as provided in paragraph (a) of this section to assume financial responsibility for any differential between the out-of-network pharmacy's (or provider's) usual and customary price and the Part D sponsor's plan allowance, consistent with the requirements of §§ 423.104(d)(2)(i)(B) and 423.104(e). (c) Limits on out-of-network access to covered Part D. A Part D sponsor must establish reasonable rules to appropriately limit out-of-network access to covered Part D drugs.