Hypervitaminosis is a condition of abnormally high storage levels of vitamins, which can lead to various symptoms as over excitement, irritability, or even toxicity. Specific medical names of the different conditions are derived from the given vitamin involved: an excess of vitamin A, for example, is called hypervitaminosis A. Hypervitaminoses are primarily caused by fat-soluble vitamins (D and A), as these are stored by the body for longer than the water-soluble vitamins.

Generally, toxic levels of vitamins stem from high supplement intake and not always from natural sources but rather the mix of natural, derived vitamins and enhancers (vitamin boosters). Toxicities of fat-soluble vitamins can also be caused by a large intake of highly fortified foods, but natural food in modest levels rarely deliver extreme or dangerous levels of fat-soluble vitamins. The Dietary Reference Intake recommendations from the United States Department of Agriculture define a "tolerable upper intake level" for most vitamins.

For those who are entirely healthy and do not experience long periods of avitaminosis, vitamin overdose can be avoided by not taking more than the normal or recommended amount of multi-vitamin supplement shown on the bottle and not ingesting multiple vitamin-containing supplements concurrently.