In the wake of warnings by the U.S. Food and Drug Administration that a commonly prescribed combination of diet pills could be linked to serious heart valve problems, doctors here said the drug was in widespread use in the Chicago area and that many people taking it shouldn't be.

They said the drug combination--fenfluramine and phentermine, or fen-phen--should be used only for the "morbidly obese," who typically suffer from a range of physical and psychological symptoms potentially more serious than the possible side effects of the drugs.

"For people who need to lose 10 or 15 pounds, you don't use medication," said Dr. Robert Kushner, medical director of the nutrition and weight control clinic at the University of Chicago. "But for people who have a serious weight problem, defined as 30 percent to 40 percent above a healthy body weight, who have other medical problems like diabetes because of that weight, and who have had difficulty following a weight- management program, that's the kind of person you ought to think about using drugs for."