Vitamin C has low toxicity and does not cause serious adverse effects at high intakes. The most common complaints are diarrhea, nausea, abdominal cramps, and other gastrointestinal disorders due to the osmotic effect of unabsorbed vitamin C in the gastrointestinal tract.
A high intake of vitamin C also has the potential to increase the excretion of oxalic and uric acid in the urine, which could contribute to the formation of kidney stones, especially in people with kidney disorders. However, studies evaluating the effects on urinary oxalate excretion from vitamin C intake ranging from 30 mg to 10 g / day have had conflicting results, so it is unclear whether vitamin C really plays a role in kidney stone growth.
Due to the enhanced absorption of iron by vitamin C, a theoretical concern is that a high intake of vitamin C may cause excessive iron absorption. In healthy people, this does not seem to be a concern. However, in people with hereditary hemochromatosis, chronic consumption of high doses of vitamin C could worsen iron overload and lead to tissue damage.
The FNB has established a UL (maximum tolerable limit) for vitamin C that applies to both food intake and supplements. Long-term intake of vitamin C above the UL may increase the risk of adverse health effects. ULs do not apply to people taking vitamin C for medical treatment, but these people should be under the supervision of a doctor.