What Happened to Fen-Phen?


Deadly Side Effects: Major Heart Dangers Stop Fen-Phen Craze

One of the most infamous FDA cases ever, fen-phen only lasted a couple years on the market in the late 1990’s. But its meteoric rise and fall affected FDA and public opinion about prescription weight loss medications for well over a decade.

Fen-phen became so popular that even early signs of danger, including bans in specific states, just drove customers across state lines to get their fen-phen fix.

Eventually, the ‘fen’ component, fenfluramine, was proved to cause major side effects, such as heart-valve abnormalities and pulmonary hypertension. Serious side effects were present in up to a third of all fen-phen patients in those clinical studies.

The controversy brought up serious concerns about the responsibilities of patients, doctors, and the FDA in minimizing adverse events. First, patients of all sizes rushed to buy a pill originally only intended for cases of severe obesity. Doctors saw a chance for easy money, and wrote too many prescriptions without reviewing the clinical studies. And the FDA lacked adequate approval and monitoring processes and couldn’t catch fen-phen before it was too late.


The manufacturer, Wyeth, has spent over $10 billion in total damages. The FDA significantly increased its standards for new drug approval in the wake of the fen-phen controversy, and didn’t approve another prescription weight loss product until 2012. The ‘phen’ component of fen-phen, phentermine, lives on as one of two active ingredients in a new diet drug called Qsymia, recently approved by the FDA.

Why do these major market failures keep happening?

With rising obesity levels and a huge demand for quick weight loss solutions, pharmaceutical manufacturers keep pushing new drug compounds through clinical trials. Prescription drugs must pass pre-clinical trials and three phases of clinical trials before receiving FDA approval. However, even after a new drug is approved for the U.S. market, clinical trials continue into Phase 4, where long-term studies search for side effects caused by extended use. These post-marketing clinical trials have led to the death of prescription diet pills like fen-phen, but not before the products cause serious harm.

LabDoor believes that consumer product safety is too important to leave up to the manufacturers, so we lead the analysis of thousands of compounds and products using independent labs. LabDoor answers the most important questions about each product: Does it work? Is it safe? What’s the cost?