Research (carried out at Copenhagen University) has suggested certain vitamin supplements do not extend life and could even lead to a premature death. A review of 67 studies found "no convincing evidence" that antioxidant supplements cut the risk of dying.
Although I eat a healthy and good diet, I was suggested to take Vitamin B12. Vitamin B-12 is a vitamin which is important for the normal functioning of the brain and nervous system. Fruits and vegetables are often a good source of vitamins. I guess eating more fruits could help to solve many problems. In large doses, some vitamins have documented side effects that tend to be more severe with a larger dosage. Overdosing from vitamin supplementation does occur. At high enough dosages some vitamins cause side effects such as nausea, diarrhea, and vomiting etc.