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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.
Other than being an effect of poor oral hygiene, bad breath can also occur as a consequence of certain things you do. For instance, taking drugs exposes you to a wide number of synthetic, chemical compounds, which are likely linked to a wide variety of side effects, including dry mouth.