But as David Saxon of the University of Colorado’s Anschutz Medical Campus explained to me, the problems with fen-phen derived from the “fen” and not the “phen”—which is to say, a different drug called fenfluramine. “Phen,” for its part, has been prescribed as a weight-loss drug for more than half a century—far longer than any GLP-1 agonist has been on the market—and has shown no clear signs of causing serious problems. Its known side effects are similar to those of Adderall, a drug that is now used by more than 40 million Americans.