Most often, bad breath originates in the mouth, from trapped food particles that are then processed by oral bacteria. The most common location for mouth-related bad breath is the back of the tongue, where large quantities of naturally occurring bacteria can thrive on food remnants, dead skin cells and post-nasal drip (mucus coming down your throat from the nose). The waste products of these bacteria include volatile sulfur compounds (VSCs), which have a smell resembling rotten eggs. Other places where bacteria and food particles can be trapped are between the teeth, beneath the gums, and in oral appliances or dentures. Poor oral hygiene sets the stage for these problems, as well as for tooth decay and gum disease, which can also cause foul odors.