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1 Persistent bad breath or a bad taste in the mouth can also be one of the warning signs of gum disease; which is caused by plaque.  Dry mouth. This occurs when the flow of saliva decreases and can be caused by various medications, salivary gland problems or continuously breathing through the mouth.
Persistent bad breath or a bad taste in the mouth may be warning signs of gum (periodontal) disease. Gum disease is caused by the buildup of plaque on teeth. The bacteria cause toxins to form in the mouth, which irritate the gums. If gum disease continues untreated, it can damage the gums and jawbone. The medical condition dry mouth (also called xerostomia) can also cause bad breath.