Drug Substitution: The practice of replacing one prescribed drug with another that is expected to have the same clinical or psychological effect.Drugs, Generic: Drugs whose drug name is not protected by a trademark. They may be manufactured by several companies.Amino Acid Substitution: The naturally occurring or experimentally induced replacement of one or more AMINO ACIDS in a protein with another. If a functionally equivalent amino acid is substituted, the protein may retain wild-type activity. Substitution may also diminish, enhance, or eliminate protein function. Experimentally induced substitution is often used to study enzyme activities and binding site properties.