After you pay your deductible, you may still be required to pay for a share of the cost for services. This cost is called coinsurance. Coinsurance in prescription drug plans work the same way, in that you would pay a percentage of the cost (maybe 20%) of the prescription drug. Some prescription drug plans have different levels of coinsurance, and different costs for different types of drugs.
Peter was required to pay a coinsurance of 20%, so when his doctor’s visit cost $150, he paid $30 and his health plan covered the remaining $120.