Vitamin C toxicity is very rare because the body cannot store it. Amounts greater than 2,000 mg/day have been discouraged by conventional medical bodies because, for some individuals, these doses can cause stomach upset and diarrhea. However, such symptoms are not generally serious, especially if they resolve with temporary discontinuation or reduction of high-dose vitamin C supplementation.

Vitamin C is widely considered to be one of safest nutrients. However, people who have a high risk of kidney disease, kidney stones, or disorders of iron metabolism (iron overload) should avoid large doses of vitamin C and consult their doctor or nutritionist prior to taking supplementation.

With the latest RDA published in 2000, a tolerable upper intake level (UL) for vitamin C was set for the first time. A UL of 2 grams (2,000 milligrams) daily was recommended in order to prevent most adults from experiencing diarrhea and gastrointestinal disturbances. However, as stated, these symptoms are generally benign and not a sign of actual toxicity. Vitamin C is very safe.