Fen-Phen (fenfluramine + phentermine) — the Diet-Drug Craze Yanked in 1997 for Wrecking Heart Valves
When the U.S. Food and Drug Administration asked American Home Products to pull fenfluramine (Pondimin) and dexfenfluramine (Redux) from the market on 15 September 1997, it ended a weight-loss phenomenon that had been built on an off-label pairing the agency had never approved and a long-term safety record nobody had assembled. Fen-Phen — the colloquial yoking of fenfluramine, a serotonin-releasing anorectic approved in 1973, with phentermine, a stimulant approved in 1959 — had become, by 1996, one of the most prescribed drug ideas in America: roughly 18 million prescriptions in that year alone and an estimated 6 million users, exposure on the order of 61 million patient-months. The gap between the promise of easy, doctor-blessed thinness and the harm was a class of damage that does not announce itself: scarred heart valves and, more rarely, lethal pulmonary hypertension.
The combination’s popularity rested on a 1992 study and on a regulatory permission slip that was never granted. The FDA had approved fenfluramine and phentermine each as standalone short-term anorectics; it had never approved their combination, nor either drug for the open-ended, cosmetic, multi-year use that fen-phen clinics dispensed. The 1996 approval of dexfenfluramine — the more potent right-handed isomer, sold as Redux and the first new prescription weight-loss drug in 23 years — poured accelerant on the market, despite an FDA advisory committee that had initially voted against it and known pulmonary-hypertension concerns in European data.
The verdict is plain at the outset. A combination the agency never sanctioned, marketed for a use it never authorized, reached millions before any long-term cardiac safety study existed; the damage was found not by the regulator or the manufacturer but by Mayo Clinic cardiologists who noticed a pattern. Their report, published in the New England Journal of Medicine on 28 August 1997, described 24 women with no prior cardiac history who had developed an unusual valvular disease — leaflets thickened with a glistening, carcinoid-like plaque — after taking fen-phen. The drug came off the market eighteen days later.
What followed was, for its era, the largest product-liability reckoning in pharmaceutical history. American Home Products (renamed Wyeth in 2002) agreed to a $3.75 billion national class settlement in 1999, then watched its total liability climb past $21 billion as tens of thousands of plaintiffs opted out, and the case became the standard byword for what happens when an off-label combination and a direct-to-consumer obesity market outrun the safety science that should have preceded them.