Optional, but Mandatory
Medicare Part D, the Prescription Drug Plan, for both Original Medicare and Medicare Advantage, is technically optional. However, there is a penalty for not enrolling, even if you do not take any drugs!
Medicare Part D is available to all people with Medicare and follows the same enrollment rules as Medicare Part B (meaning you need to enroll in the three months leading up to, the month of, and the three months after your 65th birthday).
How is Part D Covered?
Medicare Part D is provided through Medicare Prescription Drug Plans, Medicare Advantage, and other Medicare plans.
In other words, you can get a Prescription Drug Plan through Medicare Advantage or as a stand-alone plan.
The whole purpose of Part D is to limit out-of-pocket expenses for people with high prescription drug costs. Therefore, after a maximum annual deductible of $415 for prescription drugs is met (it varies by plan) and, through co-payments and co-insurance payments, out-of-pocket expenses reach a minimum of $6,550, users incur few expenses.
Bottom Line: Part D is optional, but if you do not enroll in Part D, you will have to pay a penalty. For all intents and purposes, it is best to think of Part D as being mandatory.