Ten drugs. Half the bill.

From 2013 to 2023, Medicare Part D's total spending grew by $131.8 billion. Just ten drugs account for 49.49% of that growth. Not ten drug categories, not ten manufacturers. Ten molecules. That concentration should reframe how analysts think about Medicare's cost trajectory.

Apixaban Alone Outpaced Everything Else

No single finding in this data is more striking than apixaban's trajectory. The blood thinner entered 2013 with $24.7 million in Medicare spending and ended 2023 at $17.4 billion, an increase of $17.4 billion that represents 13.18% of the entire decade's program growth. For context, that's nearly twice the share contributed by semaglutide, the GLP-1 drug that has dominated recent healthcare headlines.

Claims volume explains part of the story. Apixaban prescriptions grew from 79,490 in 2013 to 20,163,420 in 2023, a 70,436% increase. That's a drug that went from niche to ubiquitous within a single decade. When a therapy reaches that scale inside a program with fixed formulary structures, the cost consequences are enormous for every taxpayer funding Medicare.

Rivaroxaban, the other major oral anticoagulant in this class, also made the top ten, adding $5.1 billion over the same period. Apixaban's growth was more than three times larger. Two drugs in the same therapeutic class together account for roughly 17% of Medicare Part D's entire decade of spending growth.

The Drugs That Didn't Exist in 2013

Four of the top ten drugs had no recorded Medicare spending in 2013. They weren't cheap drugs that got expensive. They simply weren't there yet.

Together, these four drugs contributed 21.54% of total program growth, starting from zero. Semaglutide, empagliflozin, and dulaglutide alone account for 18.50% of the decade's growth. All three treat cardiometabolic conditions with large and growing patient populations. Empagliflozin and dulaglutide address type 2 diabetes; semaglutide now spans diabetes and obesity indications. As those patient populations age into Medicare eligibility, the spending trajectory for these drugs has no obvious ceiling.

This matters because the policy conversation around Medicare drug costs often centers on price increases for existing therapies. The data here shows that new drug launches, not just price inflation on legacy products, are a primary driver of program growth.

Older Drugs, Compounding Costs

The remaining top-ten drugs were present in 2013 but grew substantially. Lenalidomide, a cancer therapy, rose 307.5% from $1.18 billion to $4.82 billion. Adalimumab, the immunology blockbuster, grew 502.2% from $811 million to $4.88 billion. Ustekinumab, another immunology drug, grew 8,855.1% from $21.8 million to $1.95 billion.

Insulin glargine represents the floor of this group. Its 81.8% increase, from $2.37 billion to $4.31 billion, was the smallest percentage gain among the top absolute-growth drugs. Yet it still added $1.94 billion in program spending. At that baseline scale, even modest percentage growth translates into billions of dollars.

The combined picture across all ten drugs is one of concentration risk. When half of a program's decade-long cost growth flows through ten products, the program's financial trajectory becomes tightly coupled to the pricing and utilization decisions around those specific therapies. A formulary change, a biosimilar entry, or a coverage restriction on any one of these drugs carries outsized consequences for the program's overall budget, in either direction.

Semaglutide, empagliflozin, and dulaglutide had no recorded 2013 spending yet together account for 18.50% of total program growth. How much of the remaining 50.51% of growth is similarly attributable to drugs that launched after 2013, versus price increases on therapies that were already in the formulary a decade ago, is a question the aggregate numbers don't yet answer.