Medically speaking, bitterness in the mouth is a taste abnormality that is not only an oral problem, but may also be a result of other physical diseases. The causes are complex and varied, but it is not a pre-cancerous sign. Why is there bitterness in the mouth?1. Poor oral hygiene. Problems with oral hygiene can cause blood loss in the gums, periodontitis and pulpitis, and may also bring unpleasant experiences such as bad breath and bitterness in the mouth. Therefore daily morning and evening brushing, there is no really do a good brushing, very necessary. 2, life vices. Causes of mouth bitterness many brain workers due to work pressure, coupled with improper diet, lack of sleep, long-term smoking, lack of exercise and other characteristics, resulting in its gastrointestinal function sluggish, can also cause bitter mouth. 3, vitamin or trace element deficiency, lack of vitamin B12 or zinc, may also lead to bitterness in the mouth. 4, nerve damage. The sensation of the tongue needs to be transmitted to the brain through the nerves to know what it tastes like. If the head injury, injury to the nerves or brain, or even have dementia, may lead to a misunderstanding of the sense of taste. 5, other diseases of the body. Diabetes, liver disease, kidney disease, cancer can lead to abnormal taste in the mouth, or taste problems.