From building group health pricing models over the past 18 months, the single variable that has broken more trend assumptions than any other is GLP-1 utilization. National GLP-1 spending rose from $13.7 billion in 2018 to $71.7 billion in 2023, a 500% increase over five years (HFMA, May 2026). By early 2026, GLP-1 medicines accounted for roughly $132 billion, approximately 14% of all U.S. prescription drug spending and nearly one-third of total spending growth during the year. Nearly 8 out of every 100 prescriptions filled in March 2026 were for GLP-1 receptor agonists, the largest quarter-over-quarter increase in GLP-1 prescribing observed since tracking began in 2019 (Drug Topics, April 2026).

This article walks through a step-by-step GLP-1 drug trend factor framework for group health pricing actuaries: how to decompose pharmacy trend into GLP-1 and non-GLP-1 components, fit utilization growth with logistic adoption curves, select unit cost trend assumptions, apply credibility weighting, and stress-test stop-loss attachment points under multiple uptake scenarios.

14%
GLP-1 Share of Total U.S. Rx Spending (2026)
500%
GLP-1 Spend Growth, 2018 to 2023
$245/mo
CMS Medicare GLP-1 Bridge Manufacturer Price

The Carrier-Level Evidence

Plan-level data confirms the macro numbers. Blue Cross Blue Shield of Massachusetts disclosed that five GLP-1 drug manufacturers accounted for more than $300 million in pharmacy spend in 2024, roughly 20% of total pharmacy costs, with expenditure doubling year over year. BCBSMA dropped GLP-1 weight-loss coverage effective January 1, 2026, citing costs "growing at the fastest rate in more than a decade" and projecting GLP-1 spend approaching $1 billion absent the exclusion (Boston Globe, February 2026). MVP Health Care in Vermont reported GLP-1 costs rising 25 to 30 percent per quarter through 2024, nearly doubling year over year in its ACA rate filing.

Among large employers, the Business Group on Health surveyed 105 large employers in February and March 2026 and found two-thirds currently cover GLP-1s for weight management, though 10% plan to discontinue coverage in 2027 and 18% remain unsure. The Peterson-KFF Health System Tracker shows coverage among firms with 5,000 or more workers jumped from 28% in 2024 to 43% in 2025, with 66% of those firms reporting "significant" impact on prescription drug spending. One employer reported GLP-1 drugs jumping from the #32 to the #1 position in pharmacy spending after coverage was added.

Step 1: NDC-Level Trend Decomposition

The first pricing problem is mechanical: if GLP-1 claims remain pooled with the overall pharmacy book, their triple-digit growth rate inflates the base Rx trend factor applied to the entire non-GLP-1 formulary. This produces systematic overpricing of the non-GLP-1 component and obscures the true GLP-1 cost trajectory.

The fix is NDC-level filtering. Pull paid claims for the relevant experience period and tag every claim line whose NDC maps to a GLP-1 receptor agonist (semaglutide, tirzepatide, liraglutide, dulaglutide). Separate total pharmacy paid PMPM into two streams:

  • Non-GLP-1 Rx trend: Calculate the annual change in PMPM paid claims after excluding all GLP-1 NDCs. This isolates the underlying specialty and traditional drug trend, which has been running 5 to 8% annually on a net basis. Apply this factor to the non-GLP-1 component of the projected pharmacy budget.
  • GLP-1 Rx trend: Calculate the GLP-1-specific PMPM separately, decomposed into utilization (scripts per member per month) and unit cost (allowed per script). Evernorth data shows the GLP-1 net trend increase ran 210.2% in 2023 and 148.7% in 2024. Applying a blended 150% trend to the full pharmacy base would wildly overstate non-GLP-1 cost growth.

The decomposed trend produces a composite factor: (Non-GLP-1 PMPM × Non-GLP-1 Trend) + (GLP-1 PMPM × GLP-1 Trend) = Projected Total Rx PMPM. This is straightforward in principle but requires clean NDC mapping, since the same molecule (semaglutide) carries different NDCs for different brand names and dosage forms.

Step 2: Logistic Adoption Curves for Utilization

Projecting GLP-1 utilization using linear extrapolation from the last 12 months of experience will overshoot. Every drug class adoption follows a logistic (S-curve) trajectory: slow initial uptake, rapid inflection, then deceleration as the addressable population saturates. GLP-1s appear to be in the steep middle section of this curve. Ten million Americans were on GLP-1s in 2025; J.P. Morgan projects 25 to 30 million by 2030. The question for a group health pricing actuary is where a specific employer's covered population sits on that curve.

The modeling approach:

  1. Define the addressable ceiling. Using the carrier's enrollment data, calculate the proportion of covered lives with diagnosed diabetes, BMI above 30, or BMI above 27 with at least one comorbidity. Peterson-KFF estimates 34.2 million privately insured adults qualify medically for GLP-1s based on BMI criteria alone. For a specific employer group, this ceiling might range from 15% to 35% of covered adults depending on workforce demographics.
  2. Fit the logistic function. The standard form is P(t) = K / (1 + e-r(t - t0)), where K is the addressable ceiling, r is the growth rate parameter, and t0 is the inflection point. Calibrate r and t0 to the carrier's own enrollment-weighted GLP-1 prevalence data over the most recent 8 to 12 quarters.
  3. Overlay industry benchmarks. If carrier-specific data is sparse (common for mid-market groups with fewer than 2,000 lives), anchor the growth rate parameter to PBM trend surveys. WTW Rx Collaborative data shows GLP-1 PMPM climbing from $1.50 in 2019 to $27.23 in Q1 2025, an 18-fold increase. First-time anti-obesity GLP-1 prescribing rose 21.7% between December 2025 and March 2026 alone, while first-time anti-diabetic starts declined 9.8%.

The logistic model naturally decelerates the projected trend as the population approaches saturation, preventing the implausible result of utilization exceeding the eligible population. It also produces scenario sensitivity by varying the ceiling parameter: if oral semaglutide broadens the addressable population, the ceiling shifts upward and extends the period of rapid growth.

Step 3: Unit Cost Trend Selection

The utilization trend is only half the GLP-1 component. Unit cost trend requires balancing three offsetting forces:

  • Branded list price escalation. Novo Nordisk and Eli Lilly have moderated list price increases on flagship GLP-1s, but AWP still runs $900 to $1,400 per month for injectable semaglutide (Wegovy, Ozempic) and tirzepatide (Zepbound, Mounjaro). Wegovy's direct-to-consumer price has dropped to approximately $499 per month from an initial list above $1,300, though employer plan net costs after PBM rebates typically land between $400 and $700 per month (EBRI/BCBS analysis).
  • Manufacturer rebate trajectories. The CMS Medicare GLP-1 Bridge launching July 1, 2026, sets a $245 per month manufacturer price, creating a federal reference point. Commercial PBMs will use this benchmark in rebate negotiations, potentially compressing the net unit cost over the next two to three plan years. However, the bridge applies only to Medicare; commercial plans negotiate separately.
  • Biosimilar entry timing. FDA tentative approval of a generic semaglutide injection (Apotex) confirms scientific viability, but U.S. patent barriers mean market entry is not expected until 2031 to 2033 at the earliest. Pricing actuaries should not embed meaningful biosimilar savings in trend assumptions for projections shorter than five years. For international plans covering populations in Canada, India, or Brazil, earlier biosimilar availability (2027 to 2028) may apply.

A reasonable unit cost trend assumption for 2026 to 2028 projections falls between 3% and 8% annually on a net-of-rebate basis, depending on the PBM's negotiating leverage and the plan's formulary management. The CMS reference price puts downward pressure on the upper end of this range.

Step 4: Credibility-Weighted Blending

Most employer groups lack sufficient GLP-1 claim volume for full credibility. A 2,000-life group with 60 members on GLP-1 therapy generates perhaps 700 prescription claims per year across this drug class. Is that enough to let the group's own GLP-1 experience stand on its own?

Using a limited-fluctuation credibility framework with a 90% confidence level and 5% tolerance, the full credibility standard for pharmacy claims typically requires 1,082 claims (assuming a coefficient of variation of 1.0). A group with 700 GLP-1 claims achieves partial credibility of Z = √(700/1,082) ≈ 0.80. The composite GLP-1 trend becomes:

Composite Trend = Z × (Group GLP-1 Trend) + (1 - Z) × (Industry GLP-1 Trend)

For a Bühlmann credibility approach, the calculation is Z = n / (n + k), where n is the group's GLP-1 claim count and k is the ratio of the variance of hypothetical means to the expected value of process variance, estimated from a multi-group dataset. The Bühlmann framework handles the heterogeneity across employer groups better than limited-fluctuation when the actuary has access to multi-employer PBM data. In either framework, the industry complement should draw from large PBM trend surveys (Evernorth, Express Scripts, CVS Caremark) rather than CPI-Rx or AWP indices, which do not capture the mix shift toward GLP-1s.

Stop-Loss Attachment Point Stress Testing

GLP-1 claims shift the per-claimant cost distribution rightward. A member on Wegovy for 12 months at $600 per month net generates $7,200 in annual pharmacy spend before any associated medical costs. Members combining GLP-1 therapy with diabetes management, cardiovascular monitoring, and other chronic conditions can generate $15,000 to $20,000 or more in total annual claims attributable to GLP-1-adjacent care.

For stop-loss pricing, this has two direct effects on the excess loss factor (ELF):

  • Specific deductible leveraging. As the per-claimant distribution shifts rightward, the proportion of claimants exceeding a given specific deductible increases. Voya's 2025 analysis covering 2.2 million employees found claims incidence surging from 23.8 to 32.5 per 10,000 employees, with growth accelerating at every threshold: 8.1% annual growth at $100,000, 10.9% at $150,000, 13.2% at $500,000, and 15.4% at $750,000.
  • Aggregate attachment point adequacy. Under accelerated GLP-1 adoption, the plan's expected claims per member rises faster than historical trend loading. If the aggregate corridor (typically 125% of expected claims) was calibrated to pre-GLP-1 utilization levels, the probability of breaching the aggregate attachment increases materially under a scenario where GLP-1 penetration doubles over two years.

Model three scenarios: base case (current penetration with 15% annual utilization growth), moderate adoption (penetration doubles over three years), and rapid adoption (penetration triples, reflecting a coverage expansion or oral semaglutide launch). Recalculate the ELF under each scenario and compare with the stop-loss carrier's quoted rate. The analysis in our stop-loss GLP-1 contract analysis details how carriers are responding with carve-outs, lasers, and raised attachment points.

IBNR Under Rapid Utilization Acceleration

Quarter-over-quarter utilization acceleration creates a specific IBNR estimation challenge. Specialty pharmacy claims processing lags run 30 to 90 days depending on the PBM and distribution channel (retail vs. specialty vs. mail order). If GLP-1 utilization is growing 20 to 30% per quarter, a standard completion factor derived from historical pharmacy lag patterns will understate incurred claims for the most recent quarters.

The practical adjustment: calculate GLP-1-specific completion factors using the last 8 to 12 quarters of NDC-tagged lag data, then apply a utilization growth adjustment to the incomplete periods. If historical completion at 60 days is 85% but utilization grew 25% in the most recent quarter, the unadjusted completion factor understates the true incurred by the product of the growth rate and the incomplete fraction. This is the same methodology pricing actuaries use for rapidly growing lines of business in P&C, adapted for pharmacy claims.

Coverage Decision Scenario Modeling

The final pricing variable is the employer's own coverage decision. Two-thirds of large employers currently cover GLP-1s for weight management, but 10% expect to drop that coverage in 2027 (Business Group on Health). The actuarial consequences of each path diverge sharply:

  • Cover weight-management indication: Higher pharmacy trend, but potential downstream medical cost offsets. Aon's 18-month claims analysis found 3% medical cost growth for GLP-1 users versus 9% for non-users, though overall costs for users remain higher due to medication expense.
  • Exclude weight-management indication: Immediate pharmacy trend reduction (BCBSMA estimated roughly 3% premium savings from its exclusion). However, exclusion concentrates the remaining covered pool in higher-acuity diabetic and cardiovascular indications, potentially worsening the morbidity mix. Employers who drop coverage also face adverse selection risk: healthier members seeking GLP-1 access may leave for competitors offering coverage, degrading the risk pool.

Model both paths through a three-year projection. The cover scenario requires the full logistic adoption model and Bühlmann credibility framework above. The exclude scenario requires a morbidity pool composition adjustment, since removing weight-management-only members from the GLP-1 utilizer population changes the expected cost per remaining utilizer. Track discontinuation rates carefully: Mercer's research shows only 1 in 12 members remain on a GLP-1 for obesity at three years, with more than 50% stopping within 12 months. The cost projection for new GLP-1 cohorts is heavily front-loaded.

Why This Matters

GLP-1 drugs are the single largest pharmacy cost driver for the second consecutive year, responsible for 46.8% of total drug spend growth in 2024 (Evernorth). For pricing actuaries building group health rates, the traditional approach of applying a single pharmacy trend factor to the entire Rx book no longer produces credible results. The framework outlined here, decomposing trend at the NDC level, fitting utilization with logistic curves, selecting unit cost assumptions that account for the CMS reference price, and blending with credibility weighting, produces a more defensible and transparent rate.

The CMS Medicare GLP-1 Bridge taking effect July 1, 2026, introduces an additional variable. The $50 monthly copay and $245 manufacturer price create a federal benchmark that will reshape commercial plan negotiations over the next 12 to 24 months. Pricing actuaries who build this reference point into their trend assumptions now will avoid the lag that comes from waiting for the first renewal cycle of post-bridge PBM data.

Further Reading

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