Therapeutic equivalence in the Orange Book and AB-rated generics
Therapeutic equivalence means pharmaceutical equivalence plus proven bioequivalence. How FDA Orange Book TE codes, AB ratings and AB1/AB2 groups work.
Therapeutic equivalence in the FDA Orange Book
Therapeutic equivalence means approved pharmaceutical equivalents have demonstrated bioequivalence and are expected to have the same clinical effect and safety profile under the conditions in their labeling (FDA Orange Book preface).
FDA uses therapeutic equivalence (TE) codes to record its evaluations of multisource prescription products. The Orange Book, formally Approved Drug Products with Therapeutic Equivalence Evaluations, is in its 46th edition (2026). An AB rating is one positive code, but numbered groups such as AB1 and AB2 need separate checks.
Pharmaceutical equivalence vs bioequivalence
Pharmaceutical equivalence and bioequivalence are separate requirements. Neither alone establishes therapeutic equivalence (Orange Book preface).
Pharmaceutical equivalents contain identical amounts of the identical active ingredient, including the same salt or ester, in the identical dosage form and route of administration. They must also meet the applicable standards of strength, quality, purity and identity. Excipients don't have to match. For some products, such as reservoir or overage products and prefilled syringes, the test is identical amounts delivered over identical dosing periods (Orange Book Questions and Answers). So "same active ingredient" is a much looser idea than pharmaceutical equivalence.
Bioequivalence is about what happens in the body. It means there is no significant difference in the rate and extent to which the active ingredient becomes available, under the relevant comparison (FDA, Evaluation of Therapeutic Equivalence). Two tablets can hold the same milligrams of the same salt and still release it differently. That is why pharmaceutical equivalence alone never establishes therapeutic equivalence (Orange Book preface).
Pharmaceutical alternatives are a third category that often gets mixed in. They share the therapeutic moiety but can differ in salt, ester, strength or dosage form. Sharing a moiety does not make two products pharmaceutically or therapeutically equivalent (Orange Book preface).
| Concept | What it compares | Establishes therapeutic equivalence on its own? |
|---|---|---|
| Pharmaceutical equivalence | Active ingredient, amount, salt or ester, dosage form, route, quality standards | No |
| Bioequivalence | Rate and extent of drug availability | No |
| Therapeutic equivalence | Both of the above, plus approval, labeling and manufacturing requirements | It is the finding |
Therapeutically equivalent products are approved as safe and effective, are pharmaceutical equivalents and are bioequivalent. They also have to be adequately labeled and made under current good manufacturing practice. Appearance, excipients and packaging can differ (Orange Book preface; Orange Book Questions and Answers).
What the 80-125% bioequivalence limits mean
For immediate-release oral products meant for systemic delivery, FDA's M13A guidance compares the 90% confidence interval of the geometric mean ratio of pharmacokinetic parameters, generally Cmax and AUC, against limits of 80.00-125.00% (FDA M13A, October 2024). That is a statistical test of exposure. It does not let a generic tablet contain 80% to 125% of its labeled strength, and it doesn't apply to every dosage form or bioequivalence method.
What an AB rating means
AB means the product met the necessary bioequivalence requirements, and any actual or potential bioequivalence problem was resolved with adequate evidence (Orange Book preface). An AB-rated generic is an A-code product FDA considers therapeutically equivalent to the other pharmaceutically equivalent products in that group.
AB is not the only positive rating, and AA is another. A-family codes identify products FDA considers therapeutically equivalent to the relevant pharmaceutical equivalents. Matching only the first letter does not establish equivalence between products (Orange Book preface).
AB1, AB2 and numbered groups
FDA adds numbered suffixes, such as AB1 and AB2, to distinguish reference-product equivalence groups under a product heading. These numbers identify groups, not quality grades. AB1 is not better than AB2 (Orange Book preface).
An AB2 generic is equivalent to the AB2 reference, and nothing in the code says it is equivalent to an AB1 product. Compare the full code within the same product heading and strength. Some products carry more than one code, too (Orange Book preface).
Therapeutic equivalence codes
All codes below come from section 1.7 of the current Orange Book preface. They are categories FDA assigns. You can't work out a product's code from its dosage form.
| Code | Meaning |
|---|---|
| AA | Conventional dosage forms with no known bioequivalence problems |
| AB | Bioequivalence requirements met; actual or potential problems resolved with adequate evidence |
| AN | Qualifying solutions and powders for aerosolization (not every inhaled product) |
| AO | Injectable oil solutions with the same active ingredient, concentration and oil vehicle |
| AP | Aqueous injectable solutions and certain nonaqueous intravenous solutions |
| AT | Qualifying topical products (not every topical) |
| B* | New significant equivalence questions under investigation; FDA withholds judgment |
| BC | Extended-release products without adequate equivalence evidence |
| BD | Documented bioequivalence problems not resolved by adequate studies |
| BE | Delayed-release oral products lacking the needed evidence |
| BN | Specified aerosol or nebulizer delivery systems without established equivalence |
| BP | Potential bioequivalence problems |
| BR | Systemically acting suppositories or enemas lacking the needed evidence |
| BS | Drug standard deficiencies prevent an equivalence evaluation |
| BT | Topicals with unresolved bioequivalence issues |
| BX | Data reviewed are not enough to determine therapeutic equivalence |
What B codes and blank codes mean
A B code means therapeutic equivalence is not currently established. It doesn't mean the product is unsafe or unapproved (FDA, Evaluation of Therapeutic Equivalence).
For BX products, FDA presumes inequivalence until an adequate evaluation is done. The code indicates insufficient evidence for an equivalence determination. It is not a finding of clinical failure or a withdrawal of approval (FDA, Evaluation of Therapeutic Equivalence).
A blank TE code is a different thing again. The product may be single-source, or on the discontinued list. It may be a 505(b)(2) product with no requested or completed evaluation, a withdrawn request, or an adverse determination (FDA, Evaluation of Therapeutic Equivalence). You can't tell which from the blank alone.
One more trap: not every approved generic is pharmaceutically equivalent to its reference listed drug. A petitioned ANDA can be approved with permitted differences that rule out pharmaceutical equivalence (Orange Book Questions and Answers).
Does AB rating mean a pharmacist can substitute?
Not by itself. Therapeutically equivalent products can be expected to have the same clinical effect and safety profile under the conditions in the labeling. The Orange Book evaluation is scientific advice. It does not mandate which products are bought, prescribed, dispensed or substituted (Orange Book preface).
The actual authority to substitute comes from state law and policy, which differ (FDA, Evaluation of Therapeutic Equivalence). So an AB rating is the scientific input to a substitution decision. Before relying on it, check the active ingredient, strength, dosage form, route and full TE code against the current listing.
Do not use an Orange Book TE code to justify therapeutic interchange involving a different active ingredient. FDA's therapeutic-equivalence evaluation applies to pharmaceutical equivalents (Orange Book preface).
Which version of the guidance to use
FDA finalized its Evaluation of Therapeutic Equivalence guidance in August 2026, replacing the July 2022 draft. The Orange Book landing page still labels its link "Draft, July 2022," so use the guidance page instead of that stale label. Two related bioequivalence guidances were also finalized in May 2026: Statistical Approaches to Establishing Bioequivalence and Bioequivalence Studies With Pharmacokinetic Endpoints.
Product ratings change in cumulative supplements. The supplement opened from FDA's current link was Supplement 9 (September 2026). Check that you have the latest supplement before you rely on a product listing.
Therapeutic equivalence in the UK
FDA AB status does not authorize substitution in the UK; NHS dispensing follows its own rules (SPS).
NHS Specialist Pharmacy Service guidance, updated 31 March 2026, generally supports generic prescribing. A generic prescription can be filled with any suitable generic or branded product. In primary care, a brand-name prescription normally has to be dispensed as that brand unless a Serious Shortage Protocol applies. Differences in bioavailability, release characteristics or administration devices can justify prescribing by brand (SPS).
You can ask RegAffairs AI what therapeutic equivalence means in the Orange Book and follow its citations to the FDA source text.