Therapeutic Equivalence Codes: How the FDA Determines If Generic Drugs Can Be Substituted

When you pick up a prescription at the pharmacy, you might not realize that a complex system is deciding whether the pill you’re handed is a safe swap for the brand-name drug your doctor wrote on the script. That system is the FDA’s therapeutic equivalence (TE) codes, and it’s what keeps millions of Americans from overpaying for medications every year. These codes aren’t just bureaucratic labels-they’re science-backed decisions that tell pharmacists exactly which generics can be swapped without risking your health.

What Therapeutic Equivalence Really Means

Therapeutic equivalence doesn’t just mean two drugs have the same active ingredient. It means they work the same way in your body, produce the same clinical effect, and carry the same safety profile. The FDA doesn’t guess at this. It requires proof.

For a generic drug to earn a therapeutic equivalence rating, it must pass three tests:

• Pharmaceutical equivalence: Same active ingredient, strength, dosage form (like tablet or injection), and route of administration (oral, topical, etc.) as the brand-name drug.
• Bioequivalence: The generic must deliver the same amount of medicine into your bloodstream at the same rate as the original. This is usually proven through blood tests in healthy volunteers.
• Clinical equivalence: The FDA reviews all available data to confirm there’s no reason to believe the generic will behave differently in real patients under the conditions listed in the drug’s label.

Only when all three are met does the FDA assign a therapeutic equivalence code. This system started in 1980 with the first edition of the Orange Book-officially called Approved Drug Products with Therapeutic Equivalence Evaluations. Today, it’s updated monthly and used by pharmacists, insurers, and state pharmacy boards to make substitution decisions.

The Letter System: What AB, BC, and BX Really Mean

The TE code is a one- or two-letter rating you’ll see next to each drug in the Orange Book. It’s simple but packed with meaning.

• A-rated: These are the gold standard. They’re considered interchangeable with the brand-name drug. If your script says “brand name only,” your pharmacist can still swap it out for an A-rated generic without asking.
• B-rated: These are not automatically interchangeable. The FDA has concerns-maybe the drug’s absorption is unpredictable, or it’s a complex delivery system like an inhaler or topical cream. A B rating doesn’t mean it’s unsafe. It just means more caution is needed.

But it gets more detailed. The second letter adds context:

• AB1, AB2, AB3: Used when multiple brand-name drugs (called Reference Listed Drugs) exist for the same medicine. Each gets its own AB number. You can only substitute within the same AB group.
• BC: Extended-release tablets or capsules with bioequivalence issues.
• BD: Active ingredients with documented bioequivalence problems.
• BT: Topical products (like creams or ointments) where absorption is hard to measure.
• BN: Aerosol inhalers or nebulizer products.
• BR: Suppositories or enemas meant for systemic absorption.
• BS: Products with inconsistent drug standards.
• BX: Not enough data to make a call. These are often new generics or complex products under review.

Here’s the kicker: 90% of all generic drugs in the U.S. have an A rating. That’s over 12,600 products out of 14,000 listed in the 2023 Orange Book. But the 10% with B ratings? They’re often the trickiest-and most expensive-drugs to make.

Why Some Generics Get B Ratings (Even When They Should Work)

You might wonder: if a generic has the same ingredients, why isn’t it automatically interchangeable?

The answer lies in complexity. For simple pills, it’s easy to prove bioequivalence. You give volunteers the brand and the generic, measure blood levels over time, and compare curves. If they match within strict limits, it’s approved.

But what about a cream that absorbs through the skin? Or an inhaler that delivers medicine deep into the lungs? Or a long-acting injection that releases drug slowly over weeks? These aren’t just pills with a different label. Their delivery systems matter. And right now, the tools we use to test bioequivalence aren’t good enough to capture how these products behave in real people.

That’s why drugs like certain asthma inhalers, eczema creams, or testosterone injections often get B ratings-even if they’re clinically effective. A 2022 study in the Journal of Generic Medicines found that 68% of B-rated products were for complex delivery systems. The FDA admits this is a gap. In its 2022 draft guidance, it’s pushing for better testing methods: in vitro tests, real-world data, and even patient-reported outcomes.

For now, though, if your prescription says “B-rated,” your pharmacist can’t swap it unless your doctor says it’s okay.

How Pharmacists Use TE Codes Every Day

In a busy pharmacy, time matters. A pharmacist doesn’t have hours to read clinical studies. They look at the Orange Book. They check the TE code. And they act.

• If it’s an A-rated drug, they substitute automatically. In 49 states, laws let them do this without calling the doctor.
• If it’s a B-rated drug, they either call the prescriber or leave it as-is. Some pharmacists refuse to substitute B-rated drugs entirely-just to be safe.

According to a 2022 survey of 1,200 community pharmacists, 87% said TE codes make their job easier. But 42% of physicians still don’t fully understand them. One doctor told me (anecdotally, from a medical forum) that he once prescribed a B-rated generic for a patient with depression, only to have the pharmacist refuse to fill it-because they thought “B” meant “not safe.” The patient had to wait three days for a refill.

That’s the problem: confusion. The TE code system works brilliantly for simple pills. But for complex drugs, it creates more questions than answers.

How Much Money Does This System Save?

The numbers are staggering. Generic drugs make up 90% of all prescriptions filled in the U.S. But they cost only 23% of what brand-name drugs do. In 2023, that translated to $370 billion in savings.

Therapeutic equivalence codes are the engine behind that. Without them, pharmacies couldn’t swap drugs. Insurance companies wouldn’t cover generics. Patients would pay more.

Each time a pharmacist substitutes an A-rated generic, they save an average of $35 per prescription. Multiply that by 4 billion prescriptions a year, and you get the scale. Pharmacists spend about 2.7 minutes per script checking TE codes. That’s 1.7 million hours a year. But it’s worth it. That time translates directly into lower drug costs for patients and insurers.

What’s Changing in 2026?

The FDA isn’t sitting still. In 2023, it had 1,850 Product-Specific Guidance documents-detailed roadmaps for how to prove bioequivalence for each drug. It’s also expanding its use of real-world data: patient records, claims databases, even wearable monitors that track how a drug affects someone over time.

The goal? Reduce B-rated products for complex generics by 30% by 2027. That means more substitutions, more savings, and fewer patients stuck with expensive brand-name drugs because the system couldn’t keep up.

They’re also testing new methods:

• Using in vitro testing for topical products instead of relying on messy human trials.
• Allowing data from foreign regulators (like the EMA) when U.S. data is limited.
• Adding patient-reported outcomes to the evaluation for drugs that affect quality of life-like asthma inhalers or pain creams.

One thing won’t change: the core principle. The FDA won’t approve a generic unless it’s as safe and effective as the original. The tools are evolving. The goal isn’t.

What You Should Do

If you’re a patient:

• Ask your pharmacist: “Is this an A-rated generic?” If it is, you’re getting a safe, cheaper alternative.
• If you’re switched to a B-rated drug and feel different-fatigue, dizziness, lack of effect-tell your doctor. It might not be your body. It might be the formulation.
• Don’t assume “generic” means “less effective.” Most are identical. The system works.

If you’re a prescriber:

• Check the Orange Book before writing “dispense as written” on a prescription.
• Understand that a B rating doesn’t mean the drug is bad-it means more monitoring is needed.
• Use the FDA’s free online tools to look up TE codes. It takes 30 seconds.

The system isn’t perfect. But it’s the best we have. And for millions of people, it’s the reason they can afford their medicine.