Origin of tasting disorders

Damage of the papillae on the tongue.
Usually this is temporarily.
The taste buds are replaced every 1 or 2 weeks.

Poisoning the papillae.
This is a well know side effect of some anti-biotic and medicine like the ones used by cancer, heart failure or arithmetic. When you stop using these medicines, the disorder is finished.
Amalgam, which is used for filling teeth, can cause a taste disorder too. Amalgam is used for the silver collared fillings and contains mercury. Some people say that this amalgam will release very slowly from the fillings and can cause several health problems. It is not proven to be true, but even so, nowadays dentist use mercury free fillings most of the time.

An inflammation of the tissues of the mouth (stomatitis), for example by an infection of Chlamydia. This infection can be treated with medicine so in most cases the taste disorder will disappear.

Shortage of saliva
A healthy adult can produce 1.5 litre of saliva per day! When you get older, you will produce less saliva.
A shortage of saliva is also well known by certain diseases like: Syndrom of Sjörgren. This disease also causes a loss of tear fluids. Sjögren's ("SHOW-grins") syndrome is a chronic disease in which white blood cells attack the moisture-producing glands. The hallmark symptoms are dry eyes and dry mouth, but it is a systemic disease, affecting many organs and may cause fatigue. It is one of the most prevalent autoimmune disorders.

Nerve damage
Taste disorder can also be caused by the damage of one of the nerves that gives the taste information from the tongue to the brain.
Sometimes this happens after a visit at the dentist. It may be occur because a teeth has been pulled out. The nerve could also been damaged by the needle after an injection that is used for local anaesthetic of the mouth. Or by the anaesthetic fluid itself.
It is also possible that a virus infection causes the damaging of the nerve.
Sometimes damaging of the nerve can cause phantom taste, or a burning mouth or burning tongue syndrome.

Damage of the taste centre in the brains
This can be caused by a brain tumor. But this is rare. It occurs to only 0.5 % of the patients who visit their doctor because of a taste disorder.
Damage of the taste centre can also be caused by the use of certain medicines.