Sour saliva can be the result of a number of common problems, some of them fairly short lived and others more long term. Eating certain foods, taking some types of medication, and vitamin deficiencies can all lead to sour, bad-tasting saliva. Gum disease, infected teeth, and other dental problems may be to blame. Many times a buildup of anaerobic bacteria on the back of the tongue is the culprit, a situation that can be exacerbated by post-nasal drip, chronic dry mouth, or simply a decrease in saliva production during sleep. People with heartburn may also notice a sour taste due to acid and undigested food coming back up to their mouths.
In many cases, sour saliva is a brief problem that resolves itself once the cause is no longer present. Eating certain foods that are sour or have a strong flavor may leave a sour taste for a brief time. It may be a side effect of certain medications, but should resolve once the drugs are discontinued. Deficiencies of certain vitamins such as B12 may cause sour saliva, but this can be corrected by adding a supplement to one's diet.
Dental problems are another frequent cause of sour saliva. People with gum disease, where the gum tissue becomes swollen, inflamed, and infected, may notice their saliva is sour and unpleasant. An infected, abscessed tooth may also create a sour flavor due to the excess bacteria in the mouth.
For most people, an increase in bacteria in the mouth is behind their sour saliva. This often simply occurs because less saliva is produced during sleep to break it down, leading to a sour taste and bad breath in the morning. People with sinus infections may be more prone to sour saliva since mucous on the back of the tongue provides extra protein for the bacteria to grow in. Those with chronic dry mouth may also have problems with excess bacteria in their mouths; the condition can also interfere with the function of one's taste buds, which can exacerbate the impression of a sour taste.
Heartburn may also be behind a case of sour saliva. The condition causes a reflux of stomach acid and sometimes undigested food particles into the throat and mouth, where they can mix with the saliva and turn it sour and sometimes bitter. This can be a one-time or occasional problem, or it can be ongoing for those with a chronic case.