The medical condition dry mouth (also called xerostomia) can also cause bad breath. Saliva is necessary to moisten and cleanse the mouth by neutralizing acids produced by plaque and washing away dead cells that accumulate on the tongue, gums, and cheeks. If not removed, these cells decompose and can cause bad breath. Dry mouth may be caused by the side effects of various medications, salivary gland problems, or continuous breathing through the mouth. Dentist Carlisle PA