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In the U.S., drug manufacturers must submit an abbreviated new drug application (ANDA) to the Food and Drug Administration (FDA) for approval of a generic drug. This process is shorter and less costly compared to an approval of a brand-name drug. An approved generic drug must have the same active ingredient(s), strength, dosage form, and route of administration. This is referred as pharmaceutical equivalence. Therefore, a generic drug should act similarly and provide the same clinical outcomes as the brand drug. In addition, for a generic drug to approved by the FDA, it must show that its average rate and extent of absorption into the body is within the range of 80% to 125% of the brand drug. This is known as bioequivalence. When the expiry date of the drug patent by the innovator company (Brand name drug company) has expired, a generic drug can be manufactured without a license and this pharmaceutical product is intended to be interchangeable with the innovator product (Brand name drug).


California's state law states that the pharmacy can choose to perform a generic drug substitution if there are generic drugs available. The only exceptions are when the patient demands the brand drug for various reasons, or if the doctor has signed "do not substitute" or "dispense as written". The pharmacies' computer system automatically defaults a prescribed medication to the generic version of the medication, unless there is a comment from the doctor or refusal from the patient. Even when the prescription is called in by phone and typed out by the pharmacy staff, the brand name drug always automatically switches to a generic name drug.

When a substitution is made, the pharmacist must communicate to the patient the use of the different form of medication and indicate the name of the dispensed drug product on the prescription label, unless the prescriber orders otherwise. The state of CA requires that the drug dispensed is less or no more expensive than drug prescribed. View figure 1 below for an example of California's computer prescription system.


  1. This section does not permit substitution between long-acting and short-acting forms of a medication with the same chemical ingredients or between one drug product and two or more drug product with the same chemical ingredients.

  2. Besides the substitution of a brand-name to a generic drug, a generic substitution may also occur when switching from one generic manufacturer to another, due to the availability of different generics for a brand-name drug manufactured by different companies.

Figure 1

An example of an electronic prescription with a "Do Not Substitute" box. Reference: Manage Care Magazine.2


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