Vitamin C has low toxicity and is not believed to cause serious adverse effects at high intakes [8]. The most common complaints are diarrhea, nausea, abdominal cramps, and other gastrointestinal disturbances due to the osmotic effect of unabsorbed vitamin C in the gastrointestinal tract [4,8].
In postmenopausal women with diabetes who participated in the Iowa Women's Health Study, supplemental (but not dietary) vitamin C intake (at least 300 mg/day) was significantly associated with an increased risk of cardiovascular disease mortality [58]. The mechanism for this effect, if real, is not clear and this finding is from a subgroup of patients in an epidemiological study. No such association has been observed in any other epidemiological study, so the significance of this finding is uncertain. High vitamin C intakes also have the potential to increase urinary oxalate and uric acid excretion, which could contribute to the formation of kidney stones, especially in individuals with renal disorders [8]. However, studies evaluating the effects on urinary oxalate excretion of vitamin C intakes ranging from 30 mg to 10 g/day have had conflicting results, so it is not clear whether vitamin C actually plays a role in the development of kidney stones [8,84-86]. The best evidence that vitamin C contributes to kidney stone formation is in patients with pre-existing hyperoxaluria [22].