GLP-1s Are Rewriting Employer Trend Assumptions for 2027

Mercer projects average employer health benefit cost above $18,500 per employee in 2026, a 6.7% jump it attributes partly to GLP-1-driven pharmacy growth. Aon's study of 192,000 GLP-1 users finds sustained use lowers medical cost growth by 6 percentage points at 30 months, but only for members with Type 2 diabetes.

Both figures are correct. They describe different populations on different clocks, and a single blended trend assumption cannot be right about either.

Key Takeaways

6 percentage points of lower medical cost growth at 30 months for diabetic users, widening to 9 points at 80% adherence, against 3 points at 18 months for weight-management-only users.
5.3% to 13.8% first-year premium impact from expanding coverage, per EBRI's simulation, a same-year pharmacy cost arriving before any offset accrues.
Nearly half of GLP-1 users discontinue within 12 months, concentrated among weight-management users, who therefore never reach the month at which their smaller offset begins.
$6,540 average annual drug cost per user makes utilization the sensitive input: two percentage points on a 150-life group is a roughly $20,000 swing.
Net cost per patient varies more than twofold on PBM terms alone, from about $4,800 to $8,400 for the same molecules.

Two Curves, Two Clocks

Mercer's 2026 survey found prescription drug spending up 9.4% among large employers, with 49% covering GLP-1s for weight loss against 44% in 2024. Aon's release draws on medical and pharmacy claims from more than 50 million commercial lives between July 2022 and March 2025, the first sample with enough follow-up to speak to the offset rather than only the drug cost.

EBRI's October 2025 simulation, built on MarketScan claims across a range of eligibility, adherence and cost-sharing scenarios, put the first-year premium impact of expanding coverage at 5.3% to 13.8% annually. That is a same-year cost landing on the pharmacy line.

The offset runs on a different clock. Diabetic members sustaining therapy saw medical cost growth 6 points below non-users at 30 months, widening to 9 points at 80% adherence or better. Weight-management users without a diabetes diagnosis saw 3 points at 18 months, improving to 7 with consistent use.

Thirty months is longer than average tenure at a firm, roughly four years including turnover and plan switching, which HRP's modeling flags as why employers rarely capture the full offset before a member leaves. A sponsor pricing a twelve-month renewal is looking at the front half of a curve whose payoff lands on someone else's book, which is how the same drug class is a 6.7% cost driver in one dataset and a cost reducer in another.

Pooling the Two Populations Is the Pricing Error

The diabetic cohort's offset reflects a population with existing cardiometabolic risk, where therapy displaces hospitalizations, emergency visits and complications already priced into expected claims. The weight-management cohort's baseline carried no comparable near-term acute-event risk, so the offset is smaller and arrives later.

Persistence separates them again. Nearly half of users discontinue within 12 months, and discontinuation concentrates among those using the drug for weight loss alone. A member who stops at month nine never reaches the 18-month point where even the smaller offset starts.

So a blended assumption sits between two true curves and matches neither. It overstates the expected offset for the larger, faster-churning weight-management group and understates it for the smaller, more persistent diabetic one. A plan with 70% of its GLP-1 population in the weight-management category is pricing an assumption reflecting almost none of the members generating the favorable numbers.

The fix is to segment by indication at the point of prescription rather than by drug name, and apply the diabetic offset curve only to the diagnosed subpopulation. Most plans cannot: prior authorization forms do not reliably capture indication, and pharmacy claims often lack a clean crosswalk to medical diagnosis codes.

Group size	Utilization rate	Members on GLP-1	Approx. annual gross pharmacy add
100 lives	5%	5	~$32,700
150 lives	4%	6	~$39,200
150 lives	5%	7.5	~$49,050
150 lives	6%	9	~$58,900

Calculated at $6,540 per member per year (HRP, 2026); actual net cost varies with PBM rebate structure and member adherence.

Utilization is where the money moves, because per-user cost is high enough that a small denominator shift is real. A 100-life group at 5% utilization adds roughly $32,700 of gross pharmacy spend before rebates or offset. A 150-life group moving between 4% and 6% ranges from about $39,000 to $58,900, a near-$20,000 swing on two percentage points.

Prior authorization design is the lever. Loose administration, self-attested BMI, no step therapy, no documented lifestyle trial, pushes utilization toward full clinical eligibility, which HRP puts at over 40% of privately insured adults. Documented thresholds, a failed six-month intervention and re-authorization tied to weight loss hold it several points lower.

The tail behaves differently again. A self-insured plan's trend carries GLP-1s as a PMPM add-on, rising from roughly $1.50 in 2019 to $27.23 by the first quarter of 2025 across all covered lives. Specific stop-loss reimburses only claimants breaching a $100,000 to $250,000 deductible, which GLP-1 claims rarely do alone at $4,800 to $8,400 net per year. What matters there is co-occurrence: how often a user also carries a comorbid condition that pushes total claims past attachment. Setting that trend off rising utilization alone overstates tail severity for exactly the diabetic-comorbid claimants where the drug is likeliest to prevent the catastrophic event.

A 2026-Vintage Assumption May Not Survive One Renewal

Net cost per patient already varies more than twofold across carriers on PBM contract terms and formulary placement, from about $4,800 to $8,400 for the same molecules. A single renegotiation, or a competing molecule moved to preferred tier, shifts the utilization-sensitivity arithmetic without any change in clinical utilization.

Supply is moving faster than contracts. Oral Wegovy launched in January 2026, removing the injection barrier that had held back uptake among members unwilling to self-inject, and nine further obesity-drug candidates are approaching launch with Morningstar projecting a roughly 28% price decline as competition breaks the duopoly that has priced the category since 2023, covered in the GLP-1 price-cliff analysis.

A trend built on 2026 net pricing overstates the 2027 pharmacy line if that decline arrives on schedule and understates it if manufacturing or regulatory delay pushes it into 2028. A single-point factor carried forward without a repricing checkpoint fails in one direction or the other.

The import problem is sharper in the individual market. Mid-size carriers with 80,000 individual-market lives typically hold only 12 to 18 months of GLP-1-specific experience at meaningful scale, short of a stable trend under conventional credibility standards, and net cost variance alone can move the implied PMPM contribution 3 to 6 points, as the ACA credibility gap analysis sets out.

Layering the pooling error onto that thin base compounds it. An individual-market risk pool with a different diabetic-to-weight-management mix than the employer data behind published benchmarks will not realize the same blended offset, so a carrier importing an employer trend assumption into a filing is importing another population's mix along with the number.

GLP-1s Are Rewriting Employer Trend Assumptions for 2027

Key Takeaways

Two Curves, Two Clocks

Pooling the Two Populations Is the Pricing Error

A 2026-Vintage Assumption May Not Survive One Renewal

Further Reading

Sources