A copayment is a set amount you pay whenever you fill a prescription. e.g. if your benefit states you have a $10 copay, that means you will be responsible for $10 each time you fill your prescription, regardless how much the drug actually cost. Your health insurance company pays the rest of the bill. Coinsurance is a percentage of the total cost of prescriptions you fill. E.g. if your benefit states you have a 25% coinsurance, that means you will be responsive for 25% of the cost of the drug, and your insurance company will pay the remaining 75%. If the drug costs $500 per prescription, you will pay $125 each time you fill the prescription. However if the drug costs $100 prescription, you will only pay $25 each time you fill the prescription.