Medicare Part D Explained: What It Covers, What It Costs, and the Enrollment Mistake That Follows You Forever

Patricia retired at 67 after 30 years of teaching high school biology in suburban Cleveland. She enrolled in Medicare Parts A and B exactly when her employer’s HR department told her to – three months before her 65th birthday, just to be safe. She had her red, white, and blue Medicare card. On a Tuesday in October, she handed it to the pharmacist at her CVS for the blood pressure medication she had been taking for six years. The pharmacist ran the card twice. The total was $487. “Your Medicare doesn’t cover this,” the pharmacist said. Patricia had no Part D plan – she had not known she needed one.

Original Medicare covers hospital stays and outpatient doctor visits. It does not cover the prescription drugs you fill at a pharmacy – that is what Part D is for, and it must be purchased separately.

This is the most common Medicare mistake new enrollees make. Parts A and B together look like comprehensive coverage until you try to fill a prescription.

Why Part D Exists

When Medicare was created in 1965, prescription drugs were not the cornerstone of medical treatment they are today. The original program covered hospital care and physician services because those were the primary costs seniors faced. The pharmaceutical revolution that followed – statins, blood pressure medications, diabetes drugs – created a coverage category Medicare was not built for.

Congress added Medicare Part D in 2003 through the Medicare Modernization Act, and it became available to beneficiaries in 2006. Part D is not a government plan you automatically receive. It is a program delivered through private insurance companies approved by Medicare, and you have to select and enroll in a plan yourself during your Initial Enrollment Period.

For a full overview of what the base Medicare program covers before Part D enters the picture, see this guide to what Medicare covers.

How Part D Coverage Works

Every Part D plan is structured around four phases that reset each calendar year. The first is the deductible phase: you pay the full cost of covered drugs until you have spent up to the plan’s annual deductible. In 2025, no Part D plan can set a deductible higher than $590. The second is the initial coverage phase, where you pay a copay or coinsurance per prescription and your plan pays the rest until combined spending reaches a threshold. The third phase is the coverage gap – historically called the donut hole – which has been largely closed for most drug categories through the Inflation Reduction Act. The fourth is catastrophic coverage, which kicks in after out-of-pocket spending reaches $2,000 in 2025, at which point you pay nothing for the rest of the year.

The drug formulary – the list of specific medications your plan covers – is the most important variable between Part D plans. Two plans with identical premiums may cover your prescriptions at very different cost-sharing levels. Before choosing a plan, run your specific medications through Medicare’s online Plan Finder tool to see actual out-of-pocket costs, not just headline premiums.

The Penalty That Lasts for the Rest of Your Life

Skipping Part D when you first become eligible creates a late enrollment penalty that is permanent – it gets added to your premium every month for as long as you have Part D coverage.

The penalty is calculated as 1% of the national base beneficiary premium for every month you went without creditable prescription drug coverage. In 2025, the national base beneficiary premium is $36.78. If you went 24 months without Part D or creditable employer coverage, your penalty is roughly $8.83 per month – added permanently to whatever plan premium you eventually pay. That adds up to more than $100 per year, every year, for the rest of your life.

Creditable coverage matters here. If you retire but stay on an employer or union health plan that includes prescription drug coverage comparable to Part D, that counts – and you can delay enrolling in Part D without triggering a penalty. The moment that employer coverage ends, you have a 63-day Special Enrollment Period to sign up for Part D without a penalty. Miss that window, and the permanent penalty clock starts.

What Part D Costs

Premiums vary significantly by plan and location. According to KFF data from 2024, the average monthly Part D premium for stand-alone plans runs from $40 to $55 nationally, but low-income subsidy programs through Extra Help can reduce or eliminate this cost entirely for beneficiaries who qualify. The Extra Help program is worth checking regardless of income level; the eligibility thresholds are higher than most people assume.

Beyond premiums, your actual drug costs depend on which tier your medications fall into. Tier 1 covers generic drugs at the lowest copay. Tier 2 covers preferred brand-name drugs. Tiers 3 and above cover non-preferred and specialty drugs, for which costs can be significantly higher per fill. If a medication you take regularly is on a high tier, ask your doctor whether a therapeutic equivalent on a lower tier is medically appropriate.

Medicare’s coverage gaps extend beyond prescriptions. For a look at two other significant coverage areas that Medicare beneficiaries are often surprised to discover are not included, see this article on Medicare’s dental and vision coverage gap.

When to Enroll

Your Initial Enrollment Period for Part D runs for seven months: it starts three months before the month you turn 65, includes your birthday month, and extends three months after. Enrolling in the first three months of that window means your coverage starts on the first day of your birthday month. Waiting until the last month or two can push your coverage start date to the following month.

After the Initial Enrollment Period, you can change or add Part D coverage during the Annual Enrollment Period each fall, which runs from October 15 to December 7. Coverage changes take effect January 1 of the following year.

If you are turning 65 and currently have creditable drug coverage through an employer, make a note of exactly when that coverage ends. That date triggers your Special Enrollment Period, and the 63-day window moves fast.