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Bioequivalence and Interchangeability of Generic Drugs


Daphne E. Smith Marsh

, PharmD, BC-ADM, CDCES, University of Illinois at Chicago College of Pharmacy

Reviewed/Revised Apr 2023
Topic Resources

Medications are drugs used therapeutically (that is, to treat medical conditions). When one company develops a generic version of another company's brand-name drug, the new company’s experts in drug formulation must figure out how to make the drug. It is not enough for them to simply reproduce the brand-name drug’s chemical structure or to buy the active ingredient from a chemical manufacturer. (See also Overview of Generic Drugs and Drug Naming Overview of Generic Drugs and Drug Naming Medications are drugs used therapeutically (that is, to treat medical conditions). Drugs often have several names. When a drug is first discovered, it is given a chemical name, which describes... read more .)

Although 250 milligrams (mg) of a brand-name chemical is identical to 250 mg of the same generic chemical, a 250-mg generic pill containing that chemical may or may not have the same effect in the body as a 250-mg brand-name pill. That is because everything that is used in a particular product formulation affects how it is absorbed into the bloodstream. Inactive ingredients such as coatings, stabilizers, fillers, binders, flavorings, diluents, and others are necessary to turn a chemical into a usable drug product. These ingredients may be used to

  • Provide bulk so that a tablet is large enough to handle

  • Keep a tablet from crumbling between the time it is manufactured and the time it is used

  • Help a tablet dissolve in the stomach or intestine

  • Provide a pleasant taste and color

Inactive ingredients are usually harmless substances that do not affect the body. However, because inactive ingredients can cause unusual and sometimes severe allergic reactions in a few people, one version, or brand, of a drug may be preferable to another. For example, chemicals called bisulfites (such as sodium metabisulfite), which are used as preservatives in many products, cause asthmatic allergic reactions (wheezing, shortness of breath, chest tightness) in many people. Consequently, drug products containing bisulfites are prominently labeled as such.


Manufacturers must conduct studies to determine whether their version is bioequivalent to the original drug—that is, that the generic version releases its active ingredient (the drug) into the bloodstream at virtually the same speed and in virtually the same amounts as the original drug. Because the active ingredient in the generic drug has already been shown in testing of the brand-name drug to be safe and effective, bioequivalence studies only have to show that the generic version produces virtually the same levels of drug in the blood over time and thus require only a relatively small number (24 to 36) of healthy volunteers.

Although people generally think of oral dosage forms, such as tablets, capsules, and liquids, when they think about generic prescription drugs, generic versions of other drug dosage forms, such as injections, patches, inhalers, and others, must also meet a bioequivalence standard. The U.S. Food and Drug Administration (FDA) sets bioequivalence standards for different drug dosage forms.

The manufacturer of the brand-name drug also must prove bioequivalence before a new form of an approved drug can be sold. New forms include new dosage forms or strengths of an existing brand-name drug product and any other modified form that is developed, as well as new generic drugs. Sometimes the form that was originally tested is modified for commercial reasons. For example, tablets may need to be made sturdier, flavoring or coloring may be added or changed, or inactive ingredients may be changed to increase consumer acceptance.

Evaluation and approval procedures

The U.S. Food and Drug Administration (FDA) evaluates every generic version of a drug. The FDA approves a generic drug if studies indicate that the original brand-name drug and the generic version are essentially bioequivalent. The FDA also makes sure that a new generic drug contains the appropriate amount of the active (drug) ingredient, that it is manufactured according to federal standards (Good Manufacturing Practices), and that the generic version differs from its brand-name counterpart in size, color, and shape—a legal requirement.

Interchangeability and substitution

Theoretically, any generic drug that is bioequivalent to its brand-name counterpart may be interchanged with it. For drugs that are off patent, the generic drug may be the only form available. To limit costs, many doctors write prescriptions for generic drugs whenever possible. Even if the doctor has prescribed a brand-name drug, the pharmacist may dispense a generic drug unless the doctor wrote on the prescription that no substitution can be made. Also, insurance plans and managed care organizations may require that generic drugs be prescribed and dispensed whenever possible to save money. Some insurance plans may allow a consumer to select a more expensive brand-name product prescribed by the doctor as long as the consumer pays the difference in cost. However, in some state-run programs, the consumer may not have a choice. If the doctor prescribes a generic drug, the pharmacist must dispense a generic drug. In most states, the consumer may insist on a brand-name drug even if the doctor and pharmacist recommend a generic drug.

Sometimes generic substitution may not be appropriate. For example, some available generic versions may not be precisely bioequivalent to the brand-name drug. Such generic drugs may still be used, but they may not be substituted for the brand-name product. In cases in which small differences in the amount of drug in the bloodstream can make a very large difference in the drug’s effectiveness, generic drugs are often not substituted for brand-name drugs, although bioequivalent generic products are available. Warfarin, an anticoagulant, and phenytoin, an antiseizure drug, are examples of such drugs. Finally, a generic product may not be appropriate if it contains an inactive ingredient to which the person is allergic. Thus, if a doctor specifies a brand-name drug on the prescription and the consumer wants an equivalent generic version, the consumer or pharmacist should discuss the matter with the doctor.

Drugs that must be given in very precise amounts are less likely to be interchangeable, because the difference between an effective dose and a harmful dose (the margin of safety) or ineffective dose is small. Digoxin, used to treat people with heart failure, is an example. Switching from the brand-name version of digoxin to a generic product may cause problems, because the two versions may not be sufficiently bioequivalent. However, some generic versions of digoxin have been certified as bioequivalent by the FDA. Pharmacists and doctors can answer questions about which generic drugs are interchangeable for their brand-name counterparts and which are not.

A book published by the FDA each year and updated periodically also provides guidance about which drugs are interchangeable. This book, Approved Drug Products With Therapeutic Equivalence Evaluations (also known as "the orange book" because it has a bright orange cover), is available both in print and online to anyone but is intended for use by doctors and pharmacists. (See Approved Drug Products With Therapeutic Equivalence Evaluations.)

The substitution of a generic drug can sometimes cause other problems for the consumer (see table ). A doctor may write a prescription for a brand-name product and discuss the brand-name product with the consumer. If a pharmacist dispenses an equivalent generic product and the label does not also list the reference (brand-name product), the consumer may not know how the generic product relates to the drug the doctor prescribed. To prevent this confusion, most pharmacies now include the reference brand name on the label when a generic product is substituted.

Caution should be exercised when considering switching brands of drugs that entered the market before the 1938 Federal Food, Drug, and Cosmetic Act took effect (for example, digoxin, codeine, and phenobarbital). The few drugs in this category that are still prescribed are exempt from generic drug requirements. Switching among different versions of these drugs is unwise because no standards are available by which to compare them.

Other drugs for which generic substitution may not be appropriate include drugs that are said to have a narrow margin of safety because the toxic dose is too close to the effective dose for the drug to be used safely.


More Information

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

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