Knowing that the drugs someone takes are included in a plan's formulary list doesn't tell you everything about coverage. Plans can place other restrictions on drug availability or cost; for instance, they may create drug tiers in which the copayment for brand-name drugs is more than for generic equivalents, or the copayment for one brand is more than for another.

Also, plans are permitted to do "drug substitution," in which an individual's doctor prescribes a drug but the plan covers only its generic form or a different "equivalent" drug. The plan might also require prior authorization for certain restricted drugs; or it can stipulate "step therapy," which means he must try a less expensive medicine within a particular class of drugs before the plan will pay for a more expensive one.