The U.S. Food and Drug Administration today ordered Glenmark Generics of Mahwah, N.J., and Konec Inc. of Tucson, Ariz., to stop marketing unapproved nitroglycerin tablets. The tablets are placed under the tongue to relieve chest pain or to stop a heart attack and are marketed in 0.3 mg, 0.4 mg, and 0.6 mg dosages.