In addition, the official said, the agency wants the company to do more tests to confirm that some drugs are therapeutically equivalent to brand-name products they copy.

In September the F.D.A. told doctors that the agency no longer considered the generic drug to be therapeutically equivalent to the brand-name drug.

Generic versions of brand-name drugs typically cost less, but federal officials say they are therapeutically equivalent.

Nafcillin is considered therapeutically equivalent to oxacillin, although its safety profile is somewhat different.

If efficacy and tolerability is demonstrated, however, then a simple comparison of "cost/course of treatment" can suffice for the purpose of comparing two or more therapeutically equivalent treatment alternatives.

The company said it was concerned that gradually reducing distribution would cause many patients to unnecessarily switch to other products that it said were not therapeutically equivalent to Synthroid.

You made another statement I would consider an error: You said generic drugs were chemically and therapeutically equivalent to name-brand counterparts.

The manufacturers are also fighting a provision of the Pryor bill to establish a national committee of physicians and pharmacists to decide which products are "therapeutically equivalent," that is, interchangeable.

Drug products evaluated as "therapeutically equivalent" can be expected to have equal effect and no difference when substituted for the brand name product.

The panel would impose the requirement only where alternative "therapeutically equivalent" medications were available, Mr. Hagler said.