Bad breath (halitosis) can be caused by a variety of things, including diet, medication, poor oral hygiene, and diseases or conditions such as diabetes, GERD, lactose intolerance, gum disease, and more. Treatment for bad breath depends on the cause. Medications that cause a dry mouth can trigger bad breath. In addition, certain medications, such as triamterene and paraldehyde, have bad breath as a side effect. Causes of bad breath include the following: Diet When a person does not brush or floss their teeth thoroughly, food particles that may remain in the mouth can rot and cause foul odors.