The HHS Food and Drug Administration (FDA) works with pharmaceutical companies to ensure that all drugs marketed in the United States meet specifications for identity, strength, quality, purity, and potency. Before approving a generic drug product, FDA requires many rigorous tests and procedures to ensure that the generic drug can be substituted for the brand name drug.
FDA bases evaluations of substitutability or "therapeutic equivalence" for generic drugs on scientific evaluations. By law, generic drug products must contain the identical amounts of the same active drug ingredient as the brand name product. Drug products evaluated as "therapeutically equivalent" can be expected to have equal effect and no difference when substituted for the brand name product. FDA considers drug products to be substitutable if they meet the criteria of therapeutic equivalence, even though the generic drug may differ in certain other characteristics (e.g., shape, flavor, or preservatives).
For more information on generic drugs (FAQs), please visit: http://www.fda.gov/Drugs/ResourcesForYou/Consumers/QuestionsAnswers/ucm100100.htm