The government has asked six major toothpaste and mouthwash makers to stop asserting their products prevent plaque or gum disease, saying it has no evidence to back up such claims, officials said Friday.

A Food and Drug Administration spokesman, who asked not to be named, said letters requesting labeling changes were sent to the Warner-Lambert Co. concerning Listerine mouthwash; Colgate-Palmolive Co., about Colgate MFP fluoride toothpaste and Beecham Products about Aqua-fresh fluoride toothpaste.Also receiving letters this summer were Vipont, Inc. concerning Viadent anti-plaque oral rinse; Oral Research Laboratories, Inc. about Plax dental rinse and Rydelle-Lion Inc. about Check-Up gingival toothpaste.