You can review your Evidence of Coverage (EOC) document and you may see language from your Medicare Part D plan such as:
"[the Medicare Part D plan] cannot pay for... prescriptions that are filled by pharmacies outside the United States and territories, even for a medical emergency, for example on a cruise ship."
And as noted by Medicare:
"Part D sponsors must exclude Part D drugs from qualified prescription drug coverage if they are not used and sold in the United States. In addition, Part D sponsors may only pay for drugs that satisfy the definition of Part D drug. In general, such definition requires FDA approval for sale in the United States. Therefore, even if the manufacturer has FDA approval for a drug, the version produced for foreign markets usually does not meet all of the requirements of the U.S. approval, and thus it is considered to be unapproved."
Please note, if you are traveling outside of the continental United States, but still visiting one of the U.S. Territories (such as Puerto Rico or Guam), you might be able to find one of your Medicare Part D plan's network pharmacies and use your regular Medicare Part D plan coverage.
Click here if you wish to see the full list of U.S. Territories with links to their Medicare Part D prescription drug plans.
Related Question: Is it possible to receive drug coverage for a medication purchased outside the United States in certain very exceptional situations?
Yes - in very limited situations. In 2016, the Centers for Medicare and Medicaid Services (CMS) updated their Prescription Drug Manual and added in Section 10.7 - "Drugs Purchased in Another Country":
"Part D sponsors must exclude Part D drugs from qualified prescription drug coverage if they are not sold in the United States. In addition, Part D sponsors may only pay for drugs that satisfy the definition of Part D drug. In general, such definition requires FDA approval for sale in the United States. Therefore, even if the manufacturer has FDA approval for a drug, the version produced for foreign markets usually does not meet all of the requirements of the U.S. approval, and thus it is considered to be unapproved.
[However, i]n the event of a drug shortage, in order to ensure access to critically needed drugs in conjunction with the actions taken by the FDA, CMS will accept PDE [prescription drug event] submissions with national drug codes [NDCs] for foreign versions of Part D drugs, imported under the FDA’s exercise of its enforcement discretion. PDE [prescription drug event] allowances will be limited to the specific drug product(s), conditions, and the duration of the shortage as specified by the FDA." [emphasis added]
Medicare Prescription Drug Benefit Manual
Chapter 6 – Part D Drugs and Formulary Requirements
(Rev. 18, Issued: 01-15-16, Effective: 01-15-16; Implementation: 01-15-16)