Some will no longer offer the "fen-phen" regimen. Effective immediately, physicians at Jenny Craig's weight-management programs have been instructed to no longer write fen-phen prescriptions, says Brian Luscomb, a spokesman for the La Jolla-based company. Until recently, about 500 of the company's 793 centers offered the fen-phen option.

But other weight-loss center operators are taking a wait-and-see stance. "We're waiting until our medical board evaluates the Mayo Clinic data," says Joseph DiBartolomeo, vice president of scientific affairs for Nutri / System and its affiliate, U.S. Medical Weight Loss Clinics, which together operate more than 500 weight-loss clinics nationwide. "We're giving every patient the option of coming in to see their physician and discuss it."

The Mayo Clinic report is called "junk science" in a letter sent out to patients last week by Dr. Don S. Jensen, president and medical director of Manhattan Medical Weight Control, which operates 17 centers in Southern California and one in Fresno. "The latest report out of the Mayo Clinic has many holes in it," he writes.