AB or BX? FDA's Final Therapeutic Equivalence Guidance and the New 505(b)(2) TE-Code Route

A generic drug can clear every scientific hurdle at the United States Food and Drug Administration and still fail in the pharmacy. The reason is a two letter code most sponsors never think about until it is too late. That code is the therapeutic equivalence rating, and it decides whether a pharmacist can hand your product to a patient in place of the brand at the counter.

FDA has finalised its guidance for industry titled Evaluation of Therapeutic Equivalence. The final guidance carries a Federal Register publication date of 24 August 2026 and has been available on Public Inspection since 21 August 2026. The document explains how FDA assigns therapeutic equivalence codes, how those codes appear in the Orange Book, and it answers the questions sponsors ask most often. It finalises a draft that had been open since July 2022, and it carries one substantive new mechanism that reformulated and repurposed products have waited years for: a defined route for certain 505(b)(2) applicants to request a therapeutic equivalence code.

For Canadian sponsors selling into the United States, this is not academic. Therapeutic equivalence ratings drive substitution, formulary placement, and commercial uptake. This guide explains what the codes mean, what changed in the final guidance, and the practical steps to protect your rating from the submission stage.

Executive Summary

• What happened. FDA has finalised the Evaluation of Therapeutic Equivalence guidance (Federal Register document 2026-17215, 91 FR 54721). It carries a Federal Register publication date of 24 August 2026 and has been available on Public Inspection since 21 August 2026.
• What changed. The guidance finalises the July 2022 draft (87 FR 43529). The single substantive change is a new process by which applicants of certain 505(b)(2) applications may request TE codes. Other edits are minor clarifications.
• What a TE code is. A two letter rating in the Orange Book. A first letter of A, such as AB, means FDA considers the product therapeutically equivalent and substitutable. A first letter of B, such as BX, means it is not, usually because questions remain or the data are insufficient.
• Comment window. Comments may be submitted at any time under 21 CFR 10.115(g)(5). There is no fixed deadline.
• What to do. Canadian generic (ANDA) and 505(b)(2) sponsors should confirm the TE outcome they are targeting before they finalise the application, not after approval.

What Is a Therapeutic Equivalence Code?

A therapeutic equivalence code, or TE code, is a two letter rating FDA assigns to a multisource prescription drug and publishes in the Orange Book. It tells state agencies, prescribers, and pharmacists whether FDA considers two products therapeutically equivalent, meaning they can be substituted for each other with the expectation of the same clinical effect and safety profile. The first letter carries the verdict. A means therapeutically equivalent. B means not.

FDA rates a product as therapeutically equivalent when two conditions are met. First, it must be pharmaceutically equivalent, which means the same active ingredient, dosage form, strength, and route of administration. Second, it must be bioequivalent, which means there is no significant difference in the rate and extent to which the active ingredient becomes available at the site of action. The product must also be adequately labelled and manufactured under good manufacturing practice.

Why a TE Code Decides Commercial Success

Why does it matter? Because substitution law in most United States jurisdictions ties automatic generic substitution to the Orange Book rating. A pharmacist can usually substitute an AB rated product for the brand without calling the prescriber. A product carrying a B rating cannot be substituted that way, so it has to win each prescription on its own. That single letter can be the difference between formulary access and shelf space that never moves.

Payers and pharmacy benefit managers lean on the same signal. An A rating supports interchangeability, which is what makes the generic business model work at scale. A B rating changes the model entirely, because the product now competes on price and promotion rather than on automatic substitution. Sponsors who treat the rating as a formality often discover its value only when the prescriptions do not convert.

Consider a common scenario. A sponsor wins approval for a generic solid oral dose, celebrates, and launches. Months later the sales team reports that pharmacies are not switching patients from the brand. The file review shows the product was listed with a B rating because the bioequivalence package left an open question the reviewer could not close. Nothing was wrong with the molecule. The commercial loss came entirely from how the equivalence was documented. This is why experienced teams treat the rating as a design input, not a downstream result.

What the 2026 Final Guidance Changed

What changed? The final guidance keeps the framework from the July 2022 draft and adds one substantive mechanism. FDA now describes an additional process by which applicants of certain 505(b)(2) applications may request TE codes. The remaining changes are minor edits to improve clarity and update the document. Because this is guidance rather than a rule, comments remain open at any time under 21 CFR 10.115(g)(5).

The New 505(b)(2) TE-Code Route

A 505(b)(2) application relies in part on data the applicant did not generate, often for a product that differs from an existing reference in a way a straight generic ANDA cannot capture. Historically many of these products reached approval without an A rating, which limited substitution and blunted their commercial case. The final guidance describes a process for certain 505(b)(2) applicants to request a TE code, giving reformulated and repurposed products a defined path toward the substitutability they previously lacked.

Why this matters for reformulated and repurposed products

Many valuable 505(b)(2) products are a new strength, a different salt, or a changed dosage form of an approved drug. These products can be clinically important yet commercially disadvantaged by the absence of an A rating. A defined request route means the rating question can be raised deliberately during the application rather than left to chance after approval. For sponsors weighing an ANDA against a 505(b)(2) pathway, the substitutability outcome now belongs in that decision.

What sponsors should do

Decide early whether an A rating is part of the commercial case. Document pharmaceutical equivalence and bioequivalence against a clearly identified reference. Align the labelling with the equivalence you intend to claim, and prepare the TE-code request as part of the submission strategy rather than as an afterthought. Where the product depends on a shared manufacturing line, make sure the inspection and cleaning record can defend the claim.

How FDA Actually Assigns the Rating

In practice the rating is rarely lost at the science bench. It is lost in the file. The most common reasons a product fails to earn the A rating a sponsor expected are an incomplete bioequivalence justification, a reference product mismatch, or labelling that does not line up with the equivalence claim. FDA cannot assign and defend an A rating the record does not support, so a thin file becomes a B rating or, where the data gap is wide, a BX.

A worked example makes the point. Two sponsors file against the same reference product. The first submits a bioequivalence study with a clean statistical justification, a correctly identified reference listed drug, and labelling that mirrors the equivalence claim. It earns an AB rating and substitution follows automatically. The second submits a study with a borderline confidence interval and a reference the reviewer questions. The molecule is identical, but the second product lands with a B rating and has to fight for every prescription. The difference is not chemistry. It is the strength of the file.

The building blocks FDA weighs are consistent:

• Pharmaceutical equivalence. Same active ingredient, dosage form, strength, and route.
• Bioequivalence. No significant difference in the rate and extent of absorption against the reference.
• Adequate labelling. Content that matches the product and the equivalence being claimed.
• Good manufacturing practice. A compliant, data-integral manufacturing record behind the product.

What This Means for Canadian Sponsors

Canadian sponsors pursuing an ANDA or a 505(b)(2) application should treat the therapeutic equivalence outcome as a commercial deliverable, not a label that arrives with the approval letter. That means planning the reference product and bioequivalence strategy around the rating you need, budgeting for the Orange Book listing, and confirming that United States user fees and timelines are built into the plan. The PDUFA VIII fee changes and the review timelines belong in the budget from the outset.

It also helps to read the United States pathway against the Canadian one. Health Canada is running its own generic prioritisation pilot, and sponsors who develop for both markets should map where the strategies diverge. Sponsors developing peptide generics face the same discipline, because a strong equivalence file is what protects the rating in every one of these programmes.

There is an operational layer as well. A United States launch adds import and export obligations, agent arrangements, and the same quality expectations FDA applies to any marketed drug. Sponsors who align their reference product strategy, their bioequivalence data, and their quality systems early avoid the scramble that follows a rating surprise. Where a contract manufacturer is involved, a quality agreement and a clear oversight plan protect both the approval and the rating.

Therapeutic Equivalence Compliance Checklist

• Confirm the exact TE outcome, for example AB, that your commercial model depends on before you finalise the application.
• Identify the correct reference listed drug and document why it is the right comparator.
• Build a bioequivalence justification a reviewer can defend on the record, not one that only meets the limit on average.
• Check that pharmaceutical equivalence holds: same active ingredient, dosage form, strength, and route.
• Align the labelling with the equivalence you are claiming.
• For a 505(b)(2) product, decide early whether to use the new TE-code request process and prepare the request.
• Verify the manufacturing site can support the claim under good manufacturing practice, including data integrity.
• Plan the Orange Book listing and monitor the code FDA assigns after approval.
• Track docket FDA-2022-D-0528 and submit comments if the guidance affects your programme.

Common Mistakes

• Treating the TE code as an afterthought. Sponsors optimise for approval and discover the rating only when substitution fails at the counter.
• Assuming every generic is automatically AB. A product can be approved and still receive a B rating, including BX when the data are insufficient.
• Choosing the wrong reference product. A reference mismatch undermines the equivalence claim from the start.
• Believing a 505(b)(2) product can never be substitutable. The new request route changes that for certain applications.
• Forgetting the comment window stays open. Because this is guidance, an unfavourable interpretation can still be raised at any time under 21 CFR 10.115(g)(5).

Frequently Asked Questions

What is the difference between an AB and a BX rating?

AB means FDA has resolved actual or potential bioequivalence questions with adequate evidence and considers the product substitutable with other AB products in the same rating group. BX means FDA has decided the available data are insufficient to determine therapeutic equivalence, so the product is not rated as substitutable.

Can a 505(b)(2) drug get a therapeutic equivalence code?

Yes. The August 2026 final guidance describes a new process by which applicants of certain 505(b)(2) applications may request a TE code. Before this change, 505(b)(2) products often lacked a clear route to an A rating.

Does an A rating guarantee automatic substitution everywhere?

No. Substitution is governed by each United States state, and the specific rules vary. An A rating supports substitution under most state laws, but a sponsor should confirm the rules that apply to the target market.

When does the guidance take effect and can we still comment?

The guidance carries a Federal Register publication date of 24 August 2026 and has been available on Public Inspection since 21 August 2026. Because it is guidance, comments may be submitted at any time under 21 CFR 10.115(g)(5). There is no fixed deadline.

What is the Orange Book?

The Orange Book is FDA's publication titled Approved Drug Products With Therapeutic Equivalence Evaluations. It lists approved drug products together with the therapeutic equivalence evaluations FDA has assigned.

Do TE codes apply to over the counter or biologic products?

The framework in this guidance applies to certain products approved under section 505 of the Federal Food, Drug, and Cosmetic Act, namely stand-alone new drug applications, 505(b)(2) applications, and abbreviated new drug applications. Biologics are licensed under a separate pathway and use a different interchangeability framework.

How MFLRC Can Help

MFLRC helps Canadian pharmaceutical sponsors build submissions where the therapeutic equivalence outcome is planned, not hoped for. Our regulatory affairs and licensing team supports ANDA and 505(b)(2) strategy, reference product selection, bioequivalence planning, and Orange Book listing strategy. Our quality assurance and validation specialists make sure the manufacturing record can defend the rating under good manufacturing practice. Whether you are choosing between an ANDA and a 505(b)(2) pathway or preparing a TE-code request, we help you protect the commercial value of an A rating from the first draft.

Planning a United States launch for a generic or 505(b)(2) product? MFLRC helps Canadian sponsors secure the right therapeutic equivalence rating from the submission stage. Book a consultation to map your pathway.

Conclusion

The final Evaluation of Therapeutic Equivalence guidance does not rewrite the rules, but it closes a gap that mattered. A defined TE-code route for certain 505(b)(2) applications gives reformulated and repurposed products a path to substitutability, and it reminds every sponsor that a two letter code can decide the commercial life of an approved drug. Plan the rating early, document it well, and treat the Orange Book code as the deliverable it is.

Sources and References

• FDA, Evaluation of Therapeutic Equivalence; Guidance for Industry; Availability, Federal Register document 2026-17215, 91 FR 54721 (24 August 2026)
• FDA, Evaluation of Therapeutic Equivalence; Draft Guidance for Industry, 87 FR 43529 (21 July 2022)
• FDA, Approved Drug Products With Therapeutic Equivalence Evaluations (Orange Book)
• FDA, Search for FDA Guidance Documents
• 21 CFR 10.115, Good guidance practices (eCFR)