To the Editor.
—We read with interest the Editorial by Dr Goyan1 accompanying our recent article.2 He argues that reducing undesirable product substitutions following market withdrawal of an ineffective or unsafe drug may be "an appealing theoretical concept," but not of any practicable use. He contends that lack of a statutory mandate for the Food and Drug Administration (FDA) to consider substitution effects and the "practical difficulties of knowing what physicians would substitute for the drug" prevent the FDA from educating clinicians regarding optimal substitute drugs. This argument hinges on three questions: who has information on product substitutions, who could judge the desirability of those substitutions, and what could be done with such information?Certainly, pharmaceutical manufacturers possess commercially available data on the performance of individual products and their competitors as the basis for product marketing strategy. The FDA also uses such data routinely. Medicaid programs, other insurers,