A number of causes can cause sour taste. Bad taste can also be associated with disorders which cause bad smell also. This is because smell forms a large part of taste, and so any disorder of smell will give rise to a disorder of taste also.
The main causes of smell and taste disorders are:
Medications: some medications, in particular metronidazole and other antibiotics, can cause a sour taste in the mouth. If you have started on a new medication, this might well be the cause.
Sinusitis: Sinusitis will often cause a bad taste. A small sinus somewhere may be blocked and contain pus, this will not be enough to give you other symptoms but may cause this bad taste as it drips ito the mouth.
Neurological smell disorders: People also develop taste disorders due to some neurological reasons which are not completely understood as yet. Several neurological disorders like Parkinson's disease, Alzheimer's disease, multiple sclerosis can cause taste disorders. Diabetes can also cause taste disorders, presumably since it affects the nerves.
Temporal lobe epilepsy: epileptic attacks of the temporal lobe may produce episodes of bad taste as the only sign. Such bad taste can also present during the aura stage of a full blown generalised epileptic attack.
Smoking: Smoking, bad oral hygiene, use of cocaine, etc. will cause decrease in taste or bad tastes, along with taste disorders.
Trauma: head injuries can cause lack of taste, if the nerves are completely cut, or abnormal or bad taste, if they are injured.
Tumours: Some tumours in the olfactory region of the nose or in the hippocamus of the brain, may produce lack of taste more commonly,and occasionally bad taste.
Psychogenic: Psychogenic causes of bad taste are perhaps the most common cause. They may occur due to depression, anxiety, stress, etc.
For you, a likely cause is sinusitis. You should consult an ENT surgeon, you will need a CT scan of your Paranasal sinuses and also your brain. Your doctor may also do smell and taste tests or refer you to a neurologist for the same.
Before a doctor's consultation, you can also try the following at home:
Antihistamines - Alavert 10 mg once daily (OTC)
Decongestants - Sudafed
120 mg once daily(OTC)
Xylometazoline nasal drops - Otrivin nasal drops 2 drops three times daily in both nostrils(OTC).
Another important home treatment is Menthol steam inhalation - add Menthol salts (available OTC) to a bowl of steaming water and inhale the steam two - three times daily - this will relieve the congestion.
You may also look up this site: http://www.headandneckcente r.com/articles/smell.cfm
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