Fen-Phen (Fenfluramine/Phentermine): The Diet Drug Disaster That Cost $21 Billion

When the Cure Was Worse Than the Condition

In the 1990s, the combination of fenfluramine and phentermine — popularly known as "fen-phen" — became the hottest diet drug in America. Millions of patients seeking weight loss embraced the combination, and by the mid-1990s, an estimated 6 million Americans had taken it. The drug was hailed as a breakthrough, with physicians prescribing it enthusiastically for patients struggling with obesity.

But the miracle weight-loss regimen was quietly destroying hearts.

The Devastating Side Effects

In 1997, the Mayo Clinic published a landmark case study documenting 24 patients who developed unusual heart valve disease after taking fen-phen. The damage — characterized by leaky, fibrous heart valves — was unlike anything typically seen in patients this age. Further investigation revealed that fenfluramine and its close cousin dexfenfluramine (Redux) were causing heart valve damage in a significant portion of users. The drugs were also linked to primary pulmonary hypertension (PPH), a rare and often fatal lung disease.

In September 1997, the FDA requested that Wyeth (the manufacturer) voluntarily withdraw fenfluramine and dexfenfluramine from the market. Phentermine alone remained available, as it was not implicated in the cardiac side effects.

A Historic Settlement

The litigation that followed was staggering in scale. Hundreds of thousands of users filed claims alleging heart valve damage, and Wyeth (now part of Pfizer) ultimately paid over $21 billion in settlements — one of the largest pharmaceutical settlement funds in history at the time. The fen-phen settlement became a model for large-scale pharmaceutical mass tort resolution, establishing trust funds and medical monitoring programs for affected patients.

Lessons for Pharmaceutical Safety

The fen-phen disaster was a watershed moment for drug safety oversight. It demonstrated that combination drug therapies can produce dangerous synergistic effects not seen in individual drug trials, and that off-label prescribing of drug combinations carries particular risks. The case fundamentally changed how the FDA evaluates weight-loss drugs and how courts handle mass pharmaceutical tort claims, setting legal precedents still referenced today.