Employers Are Leaving the Big Three PBMs Faster Than Ever. The Rebate Protection Most of Them Think They Already Have Doesn't Legally Apply Until 2029.

New survey data released this week shows a ten-point drop in a single year in employer use of CVS Caremark, Express Scripts, and Optum Rx, from 64.3% of surveyed employers in 2025 to 54.3% in 2026. That migration is being treated as evidence that the market is correcting itself. The more useful finding sits three paragraphs down in the same report: employers who stayed with the Big Three were more likely to believe they received 100% rebate pass-through than employers who left for PBMs that scored better on every objective transparency measure. Belief and verification are moving in opposite directions. And the federal law most plan sponsors assume now guarantees full pass-through does not bind a calendar-year plan until January 1, 2029. If you are signing or renewing a PBM contract for 2027 or 2028, the statute is not protecting you. Your contract language is the only thing that is.

The Finding Almost Everyone Skipped

The National Alliance of Healthcare Purchaser Coalitions surveyed 408 employers in May and June and published results on August 12. The headline number moved fast through the trade press: Big Three market share among respondents fell roughly ten points in a year. Of the 27 employers who changed PBMs in the past year, 20 moved to a non–Big Three vendor. Among current Big Three clients, 55.7% said they are considering a change in the next one to three years, versus 31.1% of employers already using someone else.

Here is what got less attention. When respondents were asked whether they receive 100% rebate pass-through, 56.7% of Big Three clients said yes, compared with 49.2% of clients at other PBMs. Yet on the questions that actually determine whether pass-through is real (no spread pricing, disclosure of affiliated entities, lowest-net-cost formulary construction, disclosure of all PBM compensation), the non–Big Three vendors scored better.

Read that again. The employers with the least contractual transparency reported the most confidence. Roughly 23% of Big Three clients said they were not sure what their contract required at all, compared with about 12% at other PBMs.

That is not a scandal. It is a measurement problem, and it is one you can fix in a single afternoon with documents you already own.

Why The Gap Exists

Three structural reasons, none of which involve anyone acting in bad faith.

First, "100% pass-through" is a defined term, not a plain-English promise. What passes through is whatever the contract calls a rebate. Manufacturer money that arrives labeled as a rebate aggregator administrative fee, a data or analytics fee, a formulary placement service fee, or a manufacturer-direct payment to an affiliated entity is often outside that definition, and therefore outside your guarantee, even under a contract that says 100% pass-through on page one. The Big Three each route manufacturer negotiations through a wholly owned group purchasing organization (Zinc for CVS, Ascent for Express Scripts, Emisar for Optum), and the FTC has an open line of inquiry into exactly how value gets allocated between the PBM and its own GPO. If your contract's rebate definition does not reach through to the affiliated entity, the definition is doing more work than the percentage.

Second, most employers cannot check. About 75% of survey respondents said their claims data is held by their TPA or health plan rather than by the plan itself, and when that is the case, employers generally do not have claim-level access. You cannot reconcile a rebate guarantee you cannot recompute. Confidence becomes the substitute for evidence because evidence isn't available.

Third, the effective-date confusion. CAA 2026 does mandate 100% pass-through of rebates, fees, alternative discounts, and other remuneration, remitted quarterly. But the requirement attaches to contracts entered into, renewed, or extended for plan years beginning 30 months after February 3, 2026 to January 1, 2029 for a calendar-year plan. The separate DOL proposed rule would require PBM fee disclosure to self-funded ERISA plans; it does not require pass-through. So there is a real window, running through 2028, where the market has priced in a protection that is not yet enforceable.

Think of it the way a CFO thinks about a commercial lease. The quoted rent per square foot is not the occupancy cost. CAM charges, administrative load, and pass-through operating expenses are defined in separate sections, and a tenant who negotiates hard on rent while ignoring the expense definitions can lose the entire concession. The rebate percentage is rent. The definitions are CAM.

Five Checkpoints Before You Sign Anything for 2027 or 2028

1. Read the rebate definition before you read the rebate guarantee.

Find the defined term. Ask whether it includes all manufacturer remuneration attributable to your plan's utilization, regardless of the label applied, the entity receiving it, or whether it is characterized as compensation for services. If the definition carves out administrative fees paid to affiliates, you do not have 100% pass-through, you have 100% of a subset. Request the carve-outs be enumerated in an exhibit rather than described generically.

2. Trace the affiliated-entity chain.

Ask the PBM to identify every affiliated or commonly controlled entity that receives manufacturer money touching your claims: the GPO, the specialty pharmacy, the mail pharmacy, any wholly owned formulary services company, and any offshore contracting entity. Then ask a narrower question that is harder to deflect: what percentage of total manufacturer remuneration associated with our claims was retained by an entity other than the contracting PBM in the most recent complete plan year?

3. Take custody of your own claims data.

This is the highest-leverage item on the list and the one most often deferred. Negotiate a contractual right to claim-level data, in a defined layout, at a defined cadence, at no incremental cost, surviving termination. Without it, every guarantee in the contract is self-reported. With it, a modest analytics vendor or an internal analyst can reprice your book against NADAC or an independent MAC benchmark and tell you within weeks whether your discount guarantees are being met on aggregate or on a per-claim basis, a distinction that regularly moves six figures.

4. Pull the 2029 date forward.

Since CAA 2026's pass-through mandate does not bind you until a 2029 plan year, ask for it now by contract. Language along the lines of "Effective for the initial term, PBM shall administer this agreement in a manner that complies with the pass-through and remittance requirements of CAA 2026 as though those provisions were currently effective" costs nothing to request and produces an immediately informative response. A vendor marketing itself on transparency should be indifferent. A vendor that isn't will explain why the timing matters, and that explanation is diligence.

5. Verify the switch, don't assume it.

Twenty of the 27 employers who moved went to a non–Big Three PBM. That is a directionally sound decision, but a change in logo is not a change in economics. Confirm which network the new PBM rents, which specialty pharmacy actually dispenses, and whether manufacturer negotiations are still routed through a Big Three–owned GPO. Several strong mid-market PBMs contract for rebates through third-party aggregators; that is not disqualifying, but it must be disclosed and priced, not discovered in year two.

What This Is Worth In Dollars

An illustrative example, clearly labeled as illustrative. A 3,000-employee self-funded employer with roughly 7,000 covered lives and $9.8M in annual gross prescription spend generates about 8,400 brand claims a year. At a guaranteed rebate of $520 per brand claim, the plan expects roughly $4.4M. If 6% of the manufacturer value tied to those claims is characterized as aggregator administrative and data fees falling outside the contract's rebate definition, approximately $280,000 per year never enters the guarantee calculation at all. Over a three-year term, that is roughly $840,000, on a contract the plan sponsor accurately describes as "100% pass-through."

Nothing in that scenario requires anyone to breach the agreement. The money moves exactly as the contract permits. That is the point.

Questions Worth Asking Your Broker Or Consultant This Month

• Does our current rebate definition reach manufacturer payments made to PBM-affiliated entities, and can you show me the clause?

• Do we hold claim-level data, or does our TPA? If the TPA holds it, what will it take to change that at renewal?

• Are our discount guarantees measured in aggregate or per claim, and which claims are excluded from the measurement pool?

• What is the PBM's response to a request to comply with CAA 2026 pass-through terms during the current term rather than waiting for 2029?

• If we are considering a switch, which components of the new PBM's supply chain, network, specialty dispensing, rebate contracting, are still owned by a Big Three entity?

Takeaways

The migration away from the Big Three is real, and it is concentrated among smaller employers: use fell from 69.7% to 43.8% among employers with fewer than 1,000 employees, while employers above 10,000 barely moved (75% to 72.1%). The segment moving fastest is also the segment least likely to have claims data access, dedicated benefits analytics, or specialized counsel reviewing contract definitions. That combination is where avoidable money gets left behind.

Meanwhile, employer concern about PBM compensation as a fiduciary matter has fallen since 2023, at precisely the moment CAA 2026 raised the standard and 60.6% of respondents expect cost increases of 7% or more next plan year. Attention is drifting away from the issue as the stakes rise.

Practical next steps, in order: pull your PBM contract and locate the rebate definition and the affiliated-entity disclosure section; determine who holds your claim-level data and what it would take to hold it yourself; and add the CAA 2026 early-compliance request to your renewal or RFP document. None of that requires switching vendors. All of it makes a switch, if you decide on one, defensible.