Redux was the brand name for dexfenfluramine, a prescription weight loss drug approved by the FDA in 1996 and pulled from the market just over a year later. It was withdrawn in 1997 after researchers linked it to serious heart valve damage and a rare but dangerous form of high blood pressure in the lungs. If you’re encountering this name today, it’s likely because of its role in one of the most significant drug safety scandals in modern history, or because of its connection to the infamous “fen-phen” diet pill combination.
How Redux Worked
Redux belonged to a class of drugs called substituted amphetamine derivatives. It suppressed appetite by increasing levels of serotonin, a brain chemical that influences mood, sleep, and feelings of fullness after eating. The drug worked through a dual mechanism: it triggered the release of serotonin from nerve endings in the brain while also slowing the rate at which serotonin was reabsorbed. The net effect was more serotonin activity in the brain, which made people feel less hungry and satisfied with smaller meals.
Chemically, Redux was the more potent half of an older drug called fenfluramine (sold as Pondimin). Fenfluramine is a 50/50 mix of two mirror-image molecules. Dexfenfluramine isolated the more active version, which is why it could be prescribed at lower doses and was marketed as a refined, single-drug treatment for obesity.
The Connection to Fen-Phen
Redux and fen-phen are closely related but not the same thing. Fen-phen was an off-label combination of fenfluramine (Pondimin) and phentermine, two older drugs that doctors began prescribing together in the late 1980s and early 1990s. The combination became enormously popular. Redux, approved in 1996, was designed as a standalone alternative, using the more potent form of fenfluramine without requiring phentermine. In practice, some doctors prescribed Redux with phentermine too, creating a variation sometimes called “dex-fen-phen.”
Both Redux and the original fen-phen combination were ultimately pulled for the same reasons. The fenfluramine component, whether in its original form or the refined dexfenfluramine version, was responsible for the heart and lung damage that led to the withdrawal.
Heart Valve Damage and Lung Disease
The first major warning sign came from reports of primary pulmonary arterial hypertension, a rare and life-threatening condition in which blood pressure in the arteries of the lungs rises dangerously high. Research published in the European Respiratory Journal found that using appetite suppressants like dexfenfluramine for more than three months increased the odds of developing this condition by a factor of 23. For context, pulmonary arterial hypertension normally affects roughly one or two people per million each year, so even a large increase in relative risk translated to a small but real number of patients developing a devastating illness.
The more widespread problem was heart valve damage. In 1997, the FDA received data showing that roughly 30% of patients taking dexfenfluramine, fenfluramine, or the fen-phen combination had abnormal heart valve function, compared to about 2% in people who hadn’t taken the drugs. The valves, which normally open and close to keep blood flowing in one direction, were thickening and leaking. A study published by the American Heart Association later refined those numbers, finding that clinically meaningful valve leakage occurred in 7.6% of dexfenfluramine patients versus 2.1% of controls. The aortic valve was most commonly affected, with 6.3% of dexfenfluramine patients showing leakage compared to 1.6% in the control group.
The damage resembled a condition previously seen only in patients with a type of tumor that floods the body with serotonin. This pointed directly to the drug’s serotonin-boosting mechanism as the cause. Excess serotonin, it turned out, could stimulate the growth of abnormal tissue on heart valves.
FDA Approval and Withdrawal
The FDA approved Redux on April 29, 1996, making it the first new weight loss drug approved in over two decades. It was cleared specifically for longer-term use in markedly obese patients, a population where the health risks of staying obese were considered high enough to justify pharmaceutical intervention.
The approval was controversial from the start. The FDA’s own advisory committee had split on the decision, and some members raised concerns about the pulmonary hypertension risk that had already been observed in Europe. But demand for effective weight loss treatments was intense, and Redux quickly became one of the most prescribed new drugs in the country.
On July 8, 1997, barely 15 months after approval, the FDA issued a public health advisory. The manufacturers voluntarily withdrew both Pondimin (fenfluramine) and Redux (dexfenfluramine) from the market by September 1997. Phentermine, the other half of the fen-phen combination, remained available because it was not linked to heart or lung problems on its own.
The Legal Fallout
The withdrawal triggered one of the largest mass tort cases in pharmaceutical history. Millions of Americans had taken fen-phen or Redux during the mid-1990s, and tens of thousands filed lawsuits claiming heart valve damage. The manufacturer of fenfluramine and dexfenfluramine ultimately agreed to a settlement fund that exceeded $13 billion, one of the largest pharmaceutical settlements ever. Courts established screening programs to identify affected patients, and many who showed valve abnormalities received compensation even without symptoms.
Weight Loss Medications Available Today
The Redux saga reshaped how the FDA evaluates weight loss drugs, leading to much stricter cardiovascular safety requirements. Six prescription medications are currently approved for long-term obesity treatment, and they work through very different mechanisms than Redux did.
The newest and most prominent are the GLP-1 based drugs. Semaglutide (Wegovy) and liraglutide (Saxenda) mimic a gut hormone that targets areas of the brain regulating appetite and food intake. Tirzepatide (Zepbound) takes this approach further by mimicking two appetite-related hormones simultaneously. These injectable medications have shown weight loss results far exceeding anything Redux achieved, with significantly better safety profiles.
Other options include orlistat, which works in the gut to reduce fat absorption rather than acting on the brain; a combination of phentermine and topiramate (Qsymia), which reduces appetite through a different pathway than serotonin; and naltrexone-bupropion (Contrave), which combines two older drugs to reduce hunger and cravings. Each carries its own set of restrictions and potential side effects, but none has shown the kind of heart valve damage that ended Redux’s brief time on the market.