Plan documents are written for compliance, not for reading. But four terms decide most of what you will actually pay in a year, and they are worth ten minutes of your time.
A copay is a flat amount per fill. Coinsurance is a percentage of the drug’s cost, which means it moves when the price moves. A deductible is what you pay before the plan starts sharing costs. The out-of-pocket maximum is the ceiling, after which the plan pays everything.
The interaction between them is where people get surprised. A plan with a low copay but a high deductible can feel expensive in January and cheap in October. A plan with coinsurance on specialty drugs can produce a bill that varies month to month for the same prescription.
If you are choosing between plans, do not compare premiums alone. Take the three or four medications you actually fill, and price them against each plan’s tiers. That exercise takes half an hour and routinely changes people’s minds.