Drug Product Selection, Substitution, and Therapeutic Interchange
What counts as substitution
Drug product selection is the pharmacist's option to dispense a lower-cost therapeutically equivalent product in place of the prescribed brand. The safe frame is narrow: same active ingredient, same strength, same dosage form, and an FDA therapeutic-equivalence rating permitting interchange. Anything else - a different salt, a different release mechanism, a different strength, a capsule for a tablet - is not a substitution and requires prescriber authorization as a new prescription.
When substitution is barred
Substitution is permissive, not mandatory, and is prohibited when the prescriber has indicated that no substitution be made (for example, a do-not-substitute or brand-medically-necessary notation) or when the patient objects. The pharmacist must pass along the price benefit, notify the patient that a substitution was made, and record on the prescription record the name and manufacturer of the product actually dispensed. Some payers will accept only the brand and require a dispense-as-written code that must match the prescriber's intent, not the pharmacist's preference.
Therapeutic interchange
Substituting within a drug class under an institutional protocol or a collaborative agreement is different from generic substitution. It requires a valid prescriber-authorized protocol or a formulary policy approved through the proper committee process, and it must be documented.
Traps
The most common error is treating a strength change or a dosage-form change as a substitution. The second is assuming substitution is required whenever a generic exists - it is not. Substituting over a prescriber's explicit do-not-substitute instruction is a dispensing error, not a cost-saving measure.
Rule of thumb: identical active ingredient, identical strength, identical dosage form, interchangeable rating - otherwise pick up the phone.