Outdated drug labeling has been blamed for the ill-fated coupling of fenfluramine and phentermine as an antiobesity treatment that eventually was linked with primary pulmonary hypertension and heart valve lesions.
Researchers from the Massachusetts Institute of Technology and the Massachusetts College of Pharmacy and Allied Health Sciences have reported that, although many physicians and pharmacists who prescribed and dispensed the combination were unaware of it, phentermine is a monoamine oxidase (MAO) inhibitor. Because MAO destroys serotonin in the body and fenfluramine stops plasma serotonin from being taken up into platelets, the popular "fen-phen" combination virtually wiped out the body's ability to control the amount of serotonin in blood plasma. Excess serotonin has been found to damage blood vessels and heart valves.
Voelker R. The Problem with Fen-Phen. JAMA. 1998;280(12):1041. doi:10.1001/jama.280.12.1041-JQU80006-2-1