Generic versus brand-name medicines: what is the same, what can differ, and what to ask

The short answer

An FDA-approved generic has the same active ingredient, strength, dosage form and route as the brand. It must meet the same quality standards, and its maker must show that it delivers the same amount of medicine into the body at the same rate. For most people, it works the same way.

What can differ is how the pill looks, its inactive ingredients (such as dyes, flavors and preservatives), the company that makes it, and the price.

A change of product is worth a call to your pharmacist or prescriber in three situations: medicines where the safe dose range is narrow, such as warfarin or levothyroxine; a known allergy to an inactive ingredient; and biologic medicines, where "biosimilars" follow different rules from generics. In every case, keep taking your medicine unless your prescriber tells you otherwise.

What a generic has to match

A brand-name medicine is the first version, sold under a trademarked name by the company that developed it. A generic is a later version, usually made by another company and sold under the name of its active ingredient.

Before the U.S. Food and Drug Administration (FDA) approves a generic, the company must show that it:

• has the same active ingredient, strength and dosage form (for example, tablet or liquid), and is taken the same way (for example, by mouth or by injection);
• matches the brand in dosage, safety, effectiveness, strength, stability and quality;
• is made under the same manufacturing standards as brand-name medicines;
• carries the same label information as the brand, with certain allowed exceptions; and
• is bioequivalent. This means it delivers the same amount of active ingredient into the bloodstream, at the same rate and to the same extent, as the brand.

A generic company does not have to repeat the brand's large clinical trials. It relies on FDA's earlier finding that the brand is safe and effective. This generally makes a generic less costly to bring to market.

"Can a generic be 20% weaker?" No. That idea comes from a misreading of the test. For most medicines, FDA compares blood levels from the generic and the brand and calculates a statistical range around the average result; the whole range must fall between 80% and 125% of the brand. The differences actually found are much smaller. A 2009 FDA review of 2,070 studies of generic medicines taken by mouth approved from 1996 to 2007 found that, on average, generics differed from the brand by about 3.5% in the total amount absorbed and about 4.4% in peak blood level. In nearly 98% of those studies, the total amount absorbed differed by less than 10%.

FDA lists the generics it rates as therapeutically equivalent in a public database called the Orange Book. FDA's position is that a product with that rating can be substituted with the full expectation of the same clinical effect and safety. Not every brand has a generic, and FDA can change a product's rating if a concern comes up.

You may also see an authorized generic. This is the brand-name product itself, sold without the brand name on the label. It may have a different color or marking, and it is sometimes sold at a lower price than the brand.

What can differ, and why it matters

FDA's Orange Book notes that when one of these differences matters for a particular person, the prescriber may require a specific product as a medical necessity.

What generics mean for cost

According to FDA, 9 out of 10 prescriptions filled in the United States are for generic drugs. Competition lowers prices. FDA reports that one generic competitor can cut prices by about 30%, and that five competitors are associated with price drops of nearly 85%. Those are market-wide figures, not a promise about what you will pay at the counter.

With insurance, your cost depends on where the medicine sits on your plan's drug list. Medicare's handbook explains that a lower tier generally costs less than a higher tier. It also warns that your copayment can go up when your plan adds a generic (or a biosimilar) but you keep taking the brand. Your pharmacist can tell you whether a lower-cost option exists.

When the difference can matter

Medicines with a narrow safe range

For a few medicines, the dose that works is close to the dose that causes harm. FDA calls these narrow therapeutic index (NTI) drugs and describes them as having "a much tighter therapeutic window." Its examples include warfarin, a blood thinner, and levothyroxine, a thyroid hormone.

FDA applies tighter quality and bioequivalence limits to generic NTI drugs and approves them as clinically substitutable for the brand. Large studies of insurance records found that people who started on generic levothyroxine or generic warfarin did about as well as people who started on the brand. Those studies looked at starting treatment, not at switching products partway through.

Some clinicians are more cautious about switches. The American Thyroid Association's 2014 treatment guideline advises keeping to one identifiable levothyroxine product (the brand, or the same generic) and generally avoiding switches. After a change of product, it advises rechecking the thyroid blood test (TSH) once levels have settled, which generally takes about six weeks. For most adults, the guideline rates the evidence behind both points as low quality. The National Library of Medicine's MedlinePlus drug information also advises not switching levothyroxine brands without first talking with your doctor or pharmacist. People taking warfarin already have a regular blood test, the INR, to check how the medicine is working.

If you take one of these medicines:

• Learn both the brand and generic names, and note the manufacturer on each refill.
• If the pill or the maker changes, tell your pharmacist or prescriber. Ask whether you need a blood test sooner than planned. Some clinicians will recheck a lab after a product change.
• Ask your pharmacy whether it can keep filling the same manufacturer's product. This is not always possible.
• Do not change your dose or stop the medicine on your own.

If you take warfarin, Anticoagulants: Bleeding Precautions and Questions to Ask Your Clinician explains the INR test and other precautions.

Allergies or sensitivities to inactive ingredients

FDA says inactive ingredients usually do not matter clinically. In a small number of people, however, they can affect how well a medicine is tolerated or absorbed. These effects are usually minor and uncommon.

If you have reacted to a color additive, flavor or preservative before:

• Ask your pharmacy to add the specific ingredient to your allergy record, not just the medicine's name.
• Ask the pharmacist to check the inactive ingredients of the exact product you are getting. A generic from one maker can differ from another maker's version.
• You can also look up a product yourself. DailyMed, a free National Library of Medicine website, posts the labeling that each company submits to FDA. A product's DailyMed page lists its inactive ingredients and describes the pill's color, shape and imprint.

Not every reaction is an allergy. Medication Side Effect, Allergy, or Intolerance: Understanding the Difference explains how to describe and record what happened.

Biologic medicines and biosimilars

Biologics are medicines made from living sources, such as cells or microorganisms. They are more complex than most pills and cannot be copied exactly. Insulin is now regulated as a biologic.

A biosimilar is a biologic that is highly similar to an original FDA-approved biologic, called the reference product, and has no clinically meaningful differences from it. FDA says a biosimilar has the same strength and dosage, is given the same way, and has the same potential side effects as its reference product. A biosimilar is not called a generic, and it follows its own approval rules.

An interchangeable biosimilar has met additional FDA requirements. Under federal law, it may be substituted for the reference product without the prescriber's involvement, which is often called pharmacy-level substitution. Whether that happens depends on state law. FDA's free Purple Book database shows whether a biologic has an approved biosimilar and whether that biosimilar is interchangeable. This describes the rules as of October 21, 2026.

Many biologics are given in a clinic or infusion center. If so, ask the nurse or doctor which product you are receiving.

How substitution works at a California pharmacy

In California, the state Business and Professions Code sets these rules:

• When a pharmacist may switch. If your prescription names a brand, the pharmacist may choose a generic with the same active ingredients, strength, quantity and dosage form. This is the pharmacist's choice, not a requirement.
• It must save you money. The pharmacist may substitute only if the generic costs you less than the prescribed product, counting any pharmacy fee.
• "Do not substitute." The pharmacist may not switch if the prescriber says "Do not substitute" (or words with the same meaning) out loud or in their own handwriting, or initials a checked "Do not substitute" box. On an electronic prescription, the prescriber can type the words or check the box.
• Who it covers. These rules apply to all prescriptions, including Medi-Cal.
• You must be told. The pharmacy must tell you about the substitution, and the label must name the product you received, unless the prescriber orders otherwise.
• What the label shows. Unless the prescriber directs otherwise, a California prescription label must show either the brand name, or the generic name together with the manufacturer. Labels from outpatient pharmacies must also describe the medicine's color, shape and imprint code, with a few exceptions, such as some newly marketed medicines. Compare that description with what is in the bottle.
• Biologics. A pharmacist may substitute only an interchangeable biosimilar, only if it costs you the same or less, and only if the prescriber has not said or written "Do not substitute." You must be told. Within five days, the pharmacy must record the product name and manufacturer where your prescriber can see it.

If you believe you need the brand for a medical reason, talk with your prescriber. They decide whether to write "Do not substitute." Even then, your plan may charge the brand price or require an exception.

Outside California, nearly every state has laws or rules on substitution, and the details vary. Ask your pharmacist how your state's rules work.

Insurance: tiers, brand prices and exceptions

A formulary is the list of drugs a plan covers. Medicare drug plans typically sort these drugs into tiers, with a lower tier generally costing less. Other plans may use tiers too; your plan's drug list shows how it works.

If you or your prescriber believe you need a drug that is not covered, or a higher-tier drug because the lower-tier options would not work or would cause harm, you can ask the plan for an exception.

• Medicare drug plans (Part D). You or your prescriber can ask. The prescriber must give a supporting statement that the preferred drugs would not work as well for you, would cause harmful effects, or both. A plan may decline to cover a brand at a cost level it reserves for generics.
• Marketplace and other plans that must cover essential health benefits. The plan must decide a standard request for a drug it does not cover within 72 hours. It must decide within 24 hours when your health is at serious risk or you are partway through treatment with a drug the plan does not cover. If the plan says no, you can ask for an independent review.
• In California, plans regulated by the Department of Managed Health Care must follow those federal exception rules for individual, small-group and large-group plans. This requirement does not apply to Medi-Cal managed care plans. If a plan denies a request for a drug not on its list, the notice must give the reason and explain how to file a grievance.

For more detail, see Formulary Design and Patient Access and Step Therapy and Exception Rights. If you are choosing a plan, How to compare two health plans beyond the premium covers checking a plan's drug list before you enroll.

Two everyday examples

Questions to ask at the pharmacy or visit

• Is this the brand or a generic? What are both names?
• Who makes this product? Is it a different maker from my last fill?
• Why does this pill look different from last time?
• Does this medicine have a narrow safe range? Should I have a blood test after a product change?
• Does this product contain an inactive ingredient I have reacted to before?
• Is a generic, an authorized generic or a biosimilar available? What would each cost me?
• What tier is this medicine on my plan, and what would I pay for the brand?
• If I need a specific product for a medical reason, will you support an exception request?
• For a biologic: is this the original product or a biosimilar, and is the biosimilar interchangeable?

For more questions to ask when a medicine is new, see Questions to Ask Before Starting a Medication. To check a bottle step by step, see How to Read a Prescription Label Correctly.

When to get help

Contact your prescriber or pharmacist promptly if you get a rash, itching, or swelling of the lips, tongue or face after a dose and none of the emergency signs above.

Call your pharmacy the same day if:

• the name or strength on the label does not match what you expected;
• the pills do not match the description on the label; or
• you find two bottles that may be the same medicine under different names. Taking both could double your dose.

Call your prescriber if:

• symptoms return, or new symptoms start, after a change of product;
• a narrow-range medicine such as warfarin or levothyroxine changes maker or appearance; or
• you are thinking about stopping a medicine, or switching to or away from the brand.

If you think a medicine is not working as it should or has caused a problem, you or your clinician can report it to FDA's MedWatch program, which accepts reports from patients and consumers.

For the policy questions behind these rules, such as why some medicines have no generic, see Generic and Biosimilar Competition.

Sources

1. Generic Drug Facts — U.S. Food and Drug Administration, content current as of November 1, 2021.
2. Generic Drugs: Questions & Answers — U.S. Food and Drug Administration, content current as of September 18, 2026.
3. FDA List of Authorized Generic Drugs — U.S. Food and Drug Administration, list updated October 5, 2026.
4. Orange Book Preface — U.S. Food and Drug Administration, content current as of January 15, 2026.
5. FDA Drug Topics: Understanding Generic Narrow Therapeutic Index Drugs — U.S. Food and Drug Administration, educational presentation by Office of Generic Drugs staff, November 2022.
6. Comparing generic and innovator drugs: a review of 12 years of bioequivalence data from the United States Food and Drug Administration — Davit BM and others, Annals of Pharmacotherapy, 2009 (abstract).
7. Generic Drugs — U.S. Food and Drug Administration, content current as of October 1, 2026.
8. Medicare & You 2027 — Centers for Medicare & Medicaid Services, 2027 edition (sections on drug coverage and exceptions).
9. Setting and Implementing Standards for Narrow Therapeutic Index Drugs — U.S. Food and Drug Administration, CDER Conversations, accessed October 21, 2026.
10. Comparative effectiveness of generic vs brand-name levothyroxine in achieving normal thyrotropin levels — Brito JP and others, JAMA Network Open, 2020 (abstract).
11. Comparative outcomes of treatment initiation with brand vs. generic warfarin in older patients — Desai RJ and others, Clinical Pharmacology & Therapeutics, 2020 (abstract).
12. Guidelines for the treatment of hypothyroidism — American Thyroid Association Task Force on Thyroid Hormone Replacement, Thyroid, December 2014.
13. Levothyroxine — MedlinePlus, U.S. National Library of Medicine, last revised February 15, 2019.
14. Warfarin — MedlinePlus, U.S. National Library of Medicine, last revised March 15, 2026.
15. About DailyMed — U.S. National Library of Medicine, accessed October 21, 2026.
16. Biosimilars Basics for Patients — U.S. Food and Drug Administration, content current as of August 1, 2024.
17. 42 U.S.C. 262, Regulation of biological products — U.S. Code as reproduced by the Legal Information Institute, accessed October 21, 2026.
18. Purple Book: Database of Licensed Biological Products — U.S. Food and Drug Administration, accessed October 21, 2026.
19. California Business and Professions Code section 4073 (generic substitution) — California Legislature, as reproduced by California.Public.Law, verified current October 19, 2026.
20. California Business and Professions Code section 4073.5 (biological product substitution) — California Legislature, as reproduced by California.Public.Law, verified current October 19, 2026.
21. California Business and Professions Code section 4076 (prescription labels) — California Legislature, as reproduced by California.Public.Law, verified current October 19, 2026.
22. Formulary (glossary) — HealthCare.gov, accessed October 21, 2026.
23. 42 CFR 423.578, Exceptions process (Medicare Part D) — Electronic Code of Federal Regulations, accessed October 21, 2026.
24. 45 CFR 156.122, Prescription drug benefits — Electronic Code of Federal Regulations, accessed October 21, 2026.
25. California Health and Safety Code section 1367.24 (nonformulary drug requests) — California Legislature, as reproduced by California.Public.Law, accessed October 21, 2026.
26. Drug allergies — MedlinePlus, U.S. National Library of Medicine, reviewed January 20, 2026.
27. California Poison Control System — California Poison Control System, accessed October 21, 2026.
28. MedWatch: The FDA Safety Information and Adverse Event Reporting Program — U.S. Food and Drug Administration, content current as of October 20, 2026.