Vitamin Overdose...

In large doses some vitamins have documented side effects, that tend to be more severe with larger dosage.

Vitamin Supplements

The likelihood of consuming too much of any vitamin from food is remote, but overdosing from vitamin supplementation does occur. At high enough dosages some vitamins cause side effects such as nausea, diarrhea, and vomiting.  When side effects emerge, recovery is often accomplished by reducing the dosage.

OD in the US

The concentrations of vitamins an individual can tolerate vary widely, and appear to be related to age and state of health. In the United States, overdose exposure to all formulations of vitamins was reported by 62,562 individuals in 2004 (nearly 80% of these exposures were in children under the age of 6), leading to 53 “major†life-threatening outcomes and 3 deaths-- a small number in comparison with the 19,250 people who died of unintentional poisoning of all kinds in the U.S. in the same year (2004).