Phentermine, the other half of the pair, is still available and is now commonly used in combination with the antidepressant Prozac to treat obesity. But Wurtman warned that the Pro-phen combination could be just as dangerous as fen-phen.

That warning alarmed obesity experts because, unlike previous studies, it implicated phentermine. Wurtman suggests that phentermine's label should have been changed long ago to say that the drug alters the blood's chemistry by inhibiting the action of an enzyme known as MAO.

The labels of fenfluramine and Prozac both carry express warnings that they should not be taken with drugs that act as MAO inhibitors.

Wurtman's claims have been furiously attacked by some diet-drug companies, already under siege from lawsuits related to the fen-phen combination. His research is ``blatant, unsupported advocacy,'' said Medeva Americas Inc.

It's ``incorrect and totally misleading,'' complained SmithKline Beecham plc. The conflict-of-interest charges arise because Wurtman is a co-developer of a competing diet drug--Redux, a form of fenfluramine.

He is also co-founder of Cambridge (Mass.)-based Interneuron Pharmaceutical Inc., which made Redux. Wurtman dismisses the criticism, pointing out that Interneuron pulled Redux from the market worldwide last year and has no interest in trying to reintroduce it. Wurtman says phentermine's makers were irresponsible in failing to uncover the problem themselves.