How Insurers Build Formularies That Exclude Cheaper Drugs
A formulary is a coverage list, but it is also a set of financial decisions made months before your prescription is written. The list does not rank drugs by sticker price. It ranks them by net cost after confidential rebates, and that single choice explains why a $30 generic can sit on a non-preferred tier while a brand costing ten times more stays preferred.
The Rebate Math Behind Formulary Tiers
In the United States, a pharmacy benefit manager (PBM) administers the drug benefit for commercial plans, self-insured employers, Medicare Part D plans, and public employee programs. The PBM negotiates with manufacturers, and those negotiations produce rebates paid back to the plan after the claim clears. The rebate terms are confidential, which is why two employers with identical workforces can face wildly different net prices for the same molecule.
Here is the part that shapes the list. Rebate percentages commonly scale with the drug's list price. A manufacturer can offer a larger percentage on an expensive brand than on a cheap generic, because the absolute dollars are larger and the margin can absorb it. A generic at roughly $30 a month has almost nothing to rebate. A brand at several hundred dollars can return a meaningful share, and that share lands on the plan's ledger.
Consider a common scenario in type 2 diabetes. A generic metformin costs perhaps $10–$30 per month, while a newer brand-name agent may list for $300–$600 per month. If the brand returns a 30–50% rebate, its net cost to the plan might fall to $150–$400, still higher than the generic but far closer than the list prices suggest. When the plan also collects a dispensing fee or administrative fee, the brand can appear cheaper on the PBM's internal ledger. The generic, with no rebate, remains at its low list price but generates no offsetting revenue. The formulary committee, reviewing net cost, may then place the brand on a preferred tier and the generic on a non-preferred tier, even though the generic is clinically appropriate and far less expensive for the patient at the counter.
The PBM's own revenue often depends on the spread between what it collects and what it pays out, not on delivering the lowest possible net cost to the plan. That distinction matters. A formulary committee can make a defensible decision for the plan and still produce a worse outcome for the person standing at the pharmacy counter.
Why Cheaper Drugs Fall Off the List
Formulary committees typically review clinical evidence and net cost together. Net cost is the figure after rebates, discounts, and fees are applied. A brand that returns a $200 rebate per fill can beat a generic on that measure even when the brand's list price is far higher. The generic is not rejected for being ineffective. It is rejected for being financially uninteresting.
So the generic moves to a non-preferred tier, or off the formulary entirely, and the patient absorbs the difference through a higher copay or coinsurance. The patient never sees the rebate. Rebates flow to the plan and, in many contracts, only partially to the employer who funds the coverage. The counter price and the plan's net price are two different numbers describing the same prescription.
Formulary exclusions have grown substantially over the past decade across major commercial plans. The pattern is not random. Exclusions cluster where a therapeutic class has both a cheap older option and a newer brand with room to rebate. A plan may exclude a generic antidepressant, for instance, while preferring a brand-name selective serotonin reuptake inhibitor (SSRI) that offers a confidential rebate. The patient who does well on the generic faces either a switch or a higher out-of-pocket cost. The clinical difference between the two may be negligible, but the financial difference to the plan is not.
Prior Authorization as a Cost Filter
Prior authorization is a utilization management process: coverage is withheld until specified criteria are met. For drugs, those criteria frequently require step therapy, meaning the patient must first try and fail a preferred agent before the requested drug is covered. The preferred agent is chosen on the same net-cost logic described above, so the required first step is sometimes the more expensive drug.
The paperwork burden falls on the prescriber's office, not the insurer. Staff fax forms, chase records, and resubmit when a claim is denied for a missing detail. Appeals can take days to weeks, and a delay is not a neutral event for someone with an active flare, an infection, or a condition where the therapeutic window is narrow.
Denials are not always wrong. Some requests are genuinely premature, and a step through a well-established first-line drug is reasonable care. The objection is narrower: when the required first step is selected because of rebate economics rather than comparative effectiveness, the clinical rationale and the financial rationale have quietly swapped places.
The Spread Pricing Problem
Spread pricing is the gap between what a PBM bills a health plan for a prescription and what it reimburses the pharmacy that dispensed it. The PBM keeps the difference. The practice is legal in most US states, and it is largely invisible to the plan paying the bill unless the contract requires a pass-through or an audit right.
Independent pharmacies feel this most acutely. When reimbursement lands below acquisition cost, the pharmacy either absorbs the loss or declines to stock the drug. That decision can push a patient toward a chain or a mail-order channel, which changes the care relationship more than the invoice suggests. Some states have passed transparency laws requiring disclosure of spread or a move to pass-through pricing, though the rules and their enforcement vary widely. In states with pass-through requirements, the PBM must bill the plan exactly what it pays the pharmacy, plus a disclosed administrative fee. Where those laws are absent or weakly enforced, the gap can be substantial—often several dollars per fill, and occasionally more than the pharmacy's entire margin on a cheap generic.
This site has covered how coverage rules can quietly exclude people who meet a clinical need, as in lung cancer screening eligibility rules. Formulary design follows a similar logic: the rule is written for a population, and the individual is an edge case.
What This Costs Patients and Plans
Patients pay more at the counter for excluded drugs, and the copay is the only price they ever see. Plans may still pay more overall than they would have without the rebate arrangement, because the rebate is calculated on an inflated list price. The discount is real, but it is measured against a number the manufacturer set. For a patient with a high-deductible plan, the entire negotiated price—not just the copay—can fall on them until the deductible is met, making the list price directly relevant to their out-of-pocket burden.
Employers who self-insure often get only partial pass-through of rebates. Contract terms decide how much reaches the plan's bottom line, and those terms are rarely public. A benefits manager comparing two PBM proposals may be comparing two different definitions of net cost.
The trade-off is genuine, and it belongs to the plan. A restrictive formulary with aggressive rebates can hold premiums down for everyone enrolled. The cost lands on the smaller group whose specific drug was excluded, and there is no mechanism at the counter that returns the savings to them.
Practical Steps for Prescribers and Patients
Ask for the net cost, not the list price. When a plan says a drug is not covered, the useful question is what the plan pays after rebate, and whether the contract allows that figure to be disclosed.
Request a formulary exception in writing, and document medical necessity for the non-preferred drug with the specific prior therapies tried, doses, and outcomes. A letter that names the failure of step therapy is harder to deny than a form with a checkbox.
Compare the cash price against the insurance copay before paying. For some generics, a discount program or cash price sits below the plan's non-preferred tier, and the pharmacist can usually check both in the same transaction.
Check your state's spread pricing and transparency rules. Requirements differ, and a state insurance department or pharmacy board is the place to confirm what applies to your plan. A related piece on discharge to waiting lists shows how coverage mechanics, not clinical need, end up deciding who gets treated first.
For prescribers, the practical move is to name the alternative explicitly on the prescription and note the reason. Formulary software responds to structured fields, and a free-text comment is often ignored by the review process.
This article is informational and does not constitute personalised medical, legal, or financial advice. Coverage rules and state regulations change; confirm details with your plan, prescriber, or state insurance department.
