Nineteen billion dollars. That's what Medicare Part D spent on GLP-1 agonists alone in 2023, up from $2.6 billion just five years earlier. At the same time, the number of claims for these drugs actually fell, from 2.6 million in 2018 to 14.1 million in 2023. Wait, that's not right: claims grew from 2.6 million to 14.1 million, but spending grew more than sevenfold. The math tells the story: each claim costs more, and there are far more of them.

The result is a drug category that has fundamentally reshaped Medicare's pharmaceutical budget in less than a decade.

A 31-to-1 Cost Gap That Keeps Widening

The starkest way to understand what's happening is to compare GLP-1 agonists to metformin, the decades-old diabetes standard that still accounts for 32.9 million claims annually in Part D. In 2023, the cost per metformin claim was $42.56. The cost per GLP-1 claim was $1,353.19, a ratio of more than 31 to 1.

That gap has grown. In 2018, GLP-1s cost $992.31 per claim versus $34.81 for metformin, a ratio of roughly 28 to 1. The newer drugs are not just expensive; they're getting relatively more expensive over time, even as metformin's per-claim cost has barely moved.

SGLT2 inhibitors tell a similar story. Spending in that category grew from $1.26 billion in 2018 to $12.2 billion in 2023, with cost per claim rising from $681.19 to $1,043.83. Together, GLP-1s and SGLT2 inhibitors now represent over $31 billion in annual Part D spending, a figure that didn't exist at meaningful scale a decade ago. For Medicare beneficiaries on fixed incomes, cost-sharing tied to these prices translates directly into out-of-pocket exposure, even with catastrophic coverage protections.

Primary Care Is Writing the Checks

Endocrinologists are the specialists most associated with diabetes management, but they account for only 16.08% of total GLP-1 spending in 2023. Family Practice alone accounts for 28.19%, the largest share of any single specialty. Add Internal Medicine's 25.18%, and more than half of all GLP-1 spending flows through primary care generalists.

Nurse Practitioners, with 40,997 prescribers and 2.4 million claims, now represent 19.21% of GLP-1 spending, nearly matching endocrinology's share with a fraction of the per-claim cost ($1,332.94 versus $1,559.21). Endocrinologists carry the highest cost per claim among major specialties, reflecting their concentration in complex cases, but their 5,704 prescribers are vastly outnumbered by the generalist workforce driving volume.

This distribution matters for program costs. When a drug category scales through primary care rather than specialty care, the ceiling on utilization is much higher. There are roughly ten times as many Family Practice prescribers as endocrinologists writing GLP-1 scripts.

The Budget Pressure Is Structural, Not Cyclical

Metformin's total cost in 2023 was $1.4 billion, barely changed from $1.09 billion in 2018 despite a modest increase in claims. GLP-1 spending over the same period grew by $16.5 billion. These two trajectories are not competing; they're diverging. Metformin remains the high-volume, low-cost backbone of diabetes pharmacotherapy, while GLP-1s and SGLT2 inhibitors have become a parallel, high-cost tier that Medicare is now funding at scale.

The fiscal pressure this creates is not a temporary spike tied to a single drug launch. GLP-1 agonists now span multiple approved indications, including type 2 diabetes, obesity, and cardiovascular risk reduction, and the pipeline for similar agents is full. Each new indication expands the eligible population under Medicare coverage rules.

Given that Family Practice and Internal Medicine together account for 53.37% of GLP-1 agonist spending, the central unresolved question for Part D budget projections is whether these prescriptions are being written primarily for diabetes management, weight loss, or cardiovascular indications, because the answer determines how many of Medicare's 67 million beneficiaries could eventually qualify.