In a commentary on the treatment of obesity by Kushner,1 reference is made to the landmark 1992 study of Weintraub,2 which was a long-term prospective study involving combination treatment with fenfluramine hydrochloride (Pondimin) tablets and phentermine resin (Ionamin) capsules. Unfortunately, the commentary errs in that it states that phentermine hydrochloride was used in the study. In point of fact, patients in this study received phentermine resin, not phentermine hydrochloride.
This difference is not trivial. In oral bioavailability and pharmacokinetic studies, phentermine resin was shown to be absorbed at an approximately 3 times slower rate than phentermine hydrochloride and had a prolonged peak concentration.3 As a result, the Food and Drug Administration's "Orange Book" classifies these drugs as 2 separate products, meaning that they are not bioequivalent and cannot be substituted for each other.4 While the clinical significance of these differences is unknown, they are sufficient for
Coyne TC. Phentermine—Resin or Salt— There Are Differences. Arch Intern Med. 1997;157(20):2381-2382. doi:10.1001/archinte.1997.00440410115015