Changes in taste can have a number of causes.
An upper respiratory tract infection may cause a temporary loss of taste. Sinusitis may cause a dripping of infected material into the throat which may cause a bad taste in the mouth. The taste change normally resolves after the infection has cleared.
Dry mouth (xerostomia) which may be due to radiotherapy, systemic disease or drugs may cause a change in taste.
Many drugs may cause a change in taste. These include antihypertensives, antihistamines and antidepressants. Usually the taste change resolves if the drug is stopped which will usually require liaising with the general practitioner.
There are a number of neurological disorders which may cause a change in taste. These include alzheimer's disease, facial palsy, head injury, multiple sclerosis and parkinson's disease.
Nutritional deficiencies such as vitamin B and zinc may cause a change in taste. These are diagnosed by a blood tests and the taste change usually resolves after the deficiency has been corrected.
Some endocrine disorders may cause a change in taste. These include addison's disease, diabetes mellitus, cushing's syndrome, hypopituitarism and hypothyroidism. If one of these conditions is suspected then blood tests and further investigations may be required.
Chronic renal failure and liver disease may cause changes in taste. These are usually diagnosed by a detailed medical history and blood tests.
Infections in the mouth which may be fungal, bacterial or viral may cause changes in taste. These are diagnosed by clinical examination and by taking swabs from the mouth. The change in taste due to infections usually resolves with appropriate treatment.
Dysgeusia. A change in taste, often described as a metallic or bitter taste may occur with stress. This is called oral dysgeusia. This may occur as part of burning mouth syndrome.