Medicare Part D is a prescription drug coverage plan. It is offered by private insurance companies and is designed to help cover the cost of prescription drugs for people who are enrolled in Original Medicare (Part A and Part B).

Enrolling in a Part D plan is optional, but it is important to note that if you don’t enroll in a Part D plan when you are first eligible, you may have to pay a penalty for the rest of your life if you decide to enroll later.

Each plan has a list of covered drugs, called a formulary, and the plan's cost and coverage can vary depending on the drugs you take and the plan you choose. Some plans may have lower monthly premium but higher copayments for the drugs, others may have higher monthly premium but lower copayments for drugs.

It's important to review the plan's formulary, costs, and coverage rules each year during the annual enrollment period (Oct 15 - Dec 7) to make sure it still meets your needs.