We were disappointed to see The Post's editorial {July 21} endorsing the gimmicky generic prescribing scheme being considered by the House as part of the Medicare drug benefit. Although The Post assumed that our industry's opposition to the provision is purely to preserve market share, there is in fact growing medical apprehension about changing patients from brand to brand of a given product, especially for key categories of drugs.

A recent issue of The Journal of Nervous and Mental Disease reported a case of a patient who had a toxic reaction when switched from a generic to a brand. Different brands of the same product often differ slightly, and, in some cases, these differences are important.

Imposition of a cumbersome federal scheme to require that Medicare prescriptions bear precise wording to make it tough for physicians to avoid substitution and keep patients on a brand -- or generic -- on which they are stabilized is bound to exacerbate medical problems. The Congressional Budget Office's questionable savings projections for the generic provision may be quickly swallowed up in medical expenses if only a small percentage of adverse effects occur because of variations in medicines.

The states all have laws regulating and even encouraging substitution, although few are as tough on doctors as the federal proposal. The Jacobs Amendment would strike the federal scheme and appropriately defer to state laws. That would prevent physicians from having to jump through hoops to follow sound medical procedures.

GERALD J. MOSSINGHOFF

President, Pharmaceutical Manufacturers Association

Washington