Final answer:
The FDA considers generic drugs to be therapeutically equivalent to their brand-name counterparts and substitutable when prescribed. Generic drugs must be proven to be interchangeable with the original drugs through scientific evidence and an Abbreviated New Drug Application (ANDA) process.
Step-by-step explanation:
The drugs that the FDA considers to be therapeutically equivalent and, therefore, substitutable where permitted by the prescriber, are C) Generic drugs. Generic drugs are chemical equivalents of name-brand drugs whose patents have expired. For the approval of a generic drug, the FDA requires scientific evidence that the generic drug is interchangeable with or therapeutically equivalent to the originally approved drug through an ANDA (Abbreviated New Drug Application). These drugs are less expensive and are designed to provide the same therapeutic effects as their brand-name counterparts.
While over-the-counter (OTC) drugs are drugs that do not require a doctor's prescription and can be sold directly to consumers, they are not necessarily the equivalent of prescription drugs. Brand-name drugs are the original products that generic drugs are based on, but they are not interchangeable by definition. Controlled substances are drugs that have potential for abuse and dependency and are strictly regulated, and can have generic versions but are a separate category concerning drug regulations.