The discharge of thick mucus from the nasal passageway into the throat, which is known as post-nasal drip, is one potential contributor to bad breath. Post-nasal drip and bad breath are related in two ways. First, post-nasal drip provides a food source and a protective barrier for bacteria that are commonly associated with producing compounds that combine with breath and make it smell foul. Second, post-nasal drip can combine with food particles to form odorous debris, which is known as tonsilloliths that resides in the throat.
Post-nasal drip and bad breath often occur simultaneously. This is because the mucus that begins to build up in the throat as a result of the post-nasal drip provides an excellent food source for bacteria. Having an excellent food source means that bacteria will thrive and survive in the back of the throat, causing their numbers to increase. Further, thick mucus can act as a protective layer to bacteria, allowing them to stay and flourish.
After feeding on the mucus, bacteria must rid themselves of any leftover waste products. These waste products are generally volatile sulfur compounds, which is sulfur that evaporates quickly at regular temperatures. Sulfur has a distinctive foul odor and when it combines with air pushed out from the lungs, it results in bad breath. Volatile sulfur compounds are only one type of waste product produced by bacteria. There are several other waste products that are associated with a wide range of rancid odors, such as putrescine, the foul smelling compound produced when meat rots, and skatole, which reportedly smells like fecal matter.
Another way that post-nasal drip and bad breath are connected is through the build-up tonsilloliths. Tonsilloliths are small, hard pieces of solid white material. They form in the small crevices that exist in the tonsils. They are made of dead cells, mucus, and bacteria, and generally give off a very foul odor. Post-nasal drip and bad breath are linked because increased mucus results in a greater chance that some mucus will get trapped in the tonsils' crevices and rancid smelling tonsilloliths will form.
Treating bad breath caused by post-nasal drip involves removing the cause of the post-nasal drip. When post-nasal drip is caused by a transitory condition, such as a cold, the bad breath should resolve when the cold resolves itself. Chronic post-nasal drip is often caused by allergies and can be treated with decongestants or antihistamines. In severe chronic cases, post-nasal drip may have to be treated with surgery to open up blocked sinus passageways.