Each Medicare prescription drug plan has its own formulary (list of covered drugs). A Medicare private drug plan generally will only help pay for drugs that are on its formulary and that you purchase at a pharmacy in the plan’s network.
Some drugs are explicitly excluded from Medicare coverage by law, including certain anti-anxiety drugs, weight loss and gain drugs, and over-the-counter drugs. While these drugs are excluded from Medicare paying for them, plans can choose to cover them.
If the drug you need is not on your plan’s list of covered drugs, as long as the drug is not excluded from Medicare coverage by law, your doctor may be able to ask for an “exception” to have your plan cover it.
Drug plans provide incentives for you to use generic drugs by offering lower out-of-pocket costs for those drugs. In addition, plans can put restrictions on drugs that are on the formulary, like requiring you to get permission before a drug will be covered ("prior authorization"). If a drug is not on the formulary or if you buy it from a pharmacy not in the plan's network, you will have to pay the full cost yourself and that will not count towards the limit for reaching catastrophic coverage ($4,550 in 2014).
Only what you pay for drugs on your plan’s list of covered drugs will count towards your out-of-pocket costs. Once you reach a certain amount in out of pocket costs, the amount will you have to pay for your prescriptions will change.