Anyone who is entitled to join Medicare Part A or enrolled in Medicare Part B can join a Medicare Part D prescription drug plan. This can be a stand-alone plan that complements Medicare Part A and Part B coverage, or it can be part of a Part C Medicare Advantage managed care plan that rolls together hospital, medical, and prescription drug insurance.

Enrolling in a Part D plan is voluntary for most people; about 55 percent of those eligible for the plans have enrolled in one. If someone doesn't enroll when first eligible for Medicare (usually at age 65) but later does join a plan, he pays a penalty of 1 percent per month on the premiums for every month he's delayed. This higher premium cost applies to any plan he enrolls in, and it's permanent.

If he receives Medicaid (Medi-Cal in California) benefits, he's automatically enrolled in a low-cost Part D plan.

Who runs the Part D drug plans?

The federal government's Medicare program sets the basic rules for Part D prescription drug plans, but private insurance companies issue the individual plans themselves. Different plans are offered in every state. Some are called stand-alone plans, meaning they cover prescription drugs only and complement separate coverage under Part A for hospital insurance and Part B for medical insurance. People who opt for a Part C Medicare Advantage managed care plan can get Part D drug coverage through that plan instead.

The specific terms of payment and coverage are set by the company issuing the Part D plan, subject only to Medicare's general rules. If someone wants to enroll in a plan, he does so directly with the insurance company that offers it, whether it is a stand-alone Part D plan or part of a Part C managed care plan.