Prescription drug plans, like those under Medicare Part D, are health insurance plans that help lower the cost of prescription medications. They can be purchased separately (stand-alone) or included as part of a comprehensive health plan (e.g., Medicare Advantage). These plans usually cover a wide range of prescription drugs, with specific lists of covered medications (formulary). 

Here's a more detailed look:

Purpose:
Prescription drug plans primarily aim to reduce the out-of-pocket costs of medications, whether through copayments, coinsurance, or other cost-sharing mechanisms. 

Medicare Part D:
This is a government program that provides prescription drug coverage for those eligible for Medicare. It's an optional benefit, and beneficiaries choose from various plans offered by private companies. 

Formulary:
Each plan has a list of covered drugs (formulary). If a specific medication isn't on the list, the plan may offer alternatives or cover it with different cost-sharing. 

Costs:
Plans usually have monthly premiums, deductibles, and copayments/coinsurance. The exact costs depend on the specific plan chosen. 

Medicare Advantage Plans:
Some Medicare Advantage plans (HMOs, PPOs) offer comprehensive coverage, including prescription drug benefits (Part D) alongside doctor and hospital coverage. 

Enrollment:
Individuals can enroll in Medicare Part D plans through various channels, including the Medicare website or by contacting a plan directly. 

Late Enrollment Penalty:
There can be a penalty for enrolling in a Part D plan later than when first eligible, unless you have other creditable drug coverage.