Monthly premiums for stand-alone Part D plans, and for Part C Medicare Advantage managed care plans that include drug coverage, run from about $10 to $75 per month, depending on which plan someone chooses and where he lives. The average cost of a plan is about $25 per month; a few plans have no premium at all. Generally, plans with the broadest coverage and lowest copayments have the highest premiums.

No plan covers every prescription drug, or even comes close. First, by law some drugs are not covered at all: these include certain sedatives, tranquilizers, sleeping pills, drugs used for weight loss or gain, and over-the-counter medications.

Medicare only requires each plan to cover two drugs -- either brand-name or generic -- in each "therapeutic class" of medications. That means that for any disease or condition, a plan covers some but not all drugs. The specific drugs a plan does cover are included in a list called a formulary. A plan pays its share only of the drugs listed on its formulary and purchased from a pharmacy -- either a store or a mail-order service -- that participates in that plan.

Unfortunately, every year each plan changes the drugs it includes in its formulary. The fact that a plan now covers all of someone's drugs doesn't mean that it will next year, which means the person who's covered has to stay on his toes. Each autumn, when all plans announce changes in their formularies for the following year, he must check to make sure his drugs will still be covered by his current plan. If not, he'll want to consider changing plans.