WASHINGTON – Federal health regulators have decided not to approve an experimental diet pill called Qnexa, which had been touted by many experts as the most promising weight-loss drug in more than a decade. The drug's maker, Vivus Inc., said in a statement Thursday that the Food and Drug Administration declined to approve the drug in its present form. The agency asked for more study results and additional information on its possible health risks, including major cardiovascular events and risks for women of childbearing potential.