Causes of bad breath include consumption of certain foods, poor dental hygiene and use of tobacco products, according to Mayo Clinic. Other causes are dry mouth, mouth infections and certain medications.
Foods that particularly affect breath odor include onions and garlic, explains Mayo Clinic. These foods cause unpleasant breath after they have been digested and reached the lungs through the bloodstream, a problem that some medications also cause. Failure to brush and floss daily contributes to the build-up of plaque, which can also cause bad breath. Finally, sufferers of dry mouth, or xerostomia, lack the saliva necessary to wash away odor-producing food particles.