Weight-loss drugs fenfluramine and dexfenfluramine, used in the fen-phen combination, caused heart valve problems in patients. Cases of primary pulmonary hypertension stopped the use of the Fen-Phen drug combination as a weight-loss aid.
Some sources say that the combination had been tested on animals (specifically, dogs and cats) and it was safe for them, others say that the individual substances had been tested on animals but they had not been extensively tested on animals in combination.
Gina Kolata reported in the New York Times of September 16, 1997, on fen-phen:
"Why weren't these problems [heart valve abnormalities] noticed before? Dieters in Europe had used Dexfenfluramine for decades. Dr. Friedman [an FDA official] said he could only speculate. No one had initially thought to examine patients' hearts, he said, because animal studies had never revealed heart abnormalities and heart valve defects are not normally associated with drug use." (emphasis added)