Vitamin B is an essential vitamin for the human body, but taking large amounts of vitamin B can cause vitamin overdose and produce the following side effects: 1. Taking large amounts of vitamin B complex may cause abnormalities in the nervous system, dizziness, mental irritability, weakness, and fatigue. 2. Taking large amounts of vitamin B complex may produce significant gastrointestinal reactions with abdominal pain, diarrhea, and poor appetite. 3. Taking large amounts of vitamin B may also cause panic, dry mouth, yellow urine, and vitamin B6 overdose may lead to fetal malformation. Although B vitamins are essential substances for the human body, they should not be taken in excess. B vitamins can basically satisfy the demand in our daily diet and do not need to be supplemented; if you need to use B vitamins for treatment of other conditions, you need to use them under the guidance of a physician.