To have Medicare help pay for your drugs, you must join a Medicare Part D plan or a Medicare Advantage plan that provides Medicare prescription drug coverage (or MAPD). You can choose and join the plan that meets your needs. If you don't use a lot of prescription drugs now, you should still consider joining. As we age, most people need prescription drugs to stay healthy. For most people, joining now means you won't have to pay a penalty if you choose to join later. Your premium will be higher if you wait to join after your Initial Enrollment Period because of the penalty.
You can join a drug plan starting on October 15 and continuing through December 7 [of each year]. In most cases, if you don't join during this period, your next chance to join will be October 15 [of the following year] and you will have to pay a penalty for the months that you were without some form of creditable prescription drug coverage. And this means you pay a higher monthly premium for as long as you have Medicare prescription drug coverage.
(Source: U.S. Department of Health & Human Services)