Because vitamin C is a water-soluble vitamin and excess amounts are excreted from the body rather than stored, vitamin C toxicity is very rare, but you can still encounter some unpleasant side effects if you take too much vitamin C.
Vitamin C Toxicity
Vitamin C toxicity is extremely rare, and there's no evidence that doses as high as 10,000 mg per day are detrimental to healthy adults in any way. Excessive vitamin C intake by pregnant women can lead to rebound scurvy in infants. It can also lead to dental decalcification and increased estrogen levels.
Healthy adults who take more than 2000 mg of vitamin C per day can experience some uncomfortable side effects, including urine acidification, upset stomach, and diarrhea. Because vitamin C increases the body's ability to absorb iron, dosages over 2000 mg per day may also cause iron overload in those with hemochromatosis or thalassemia.
Because oxalate, a metabolite of vitamin C, is a constituent of kidney stones, some experts believe that high doses of vitamin C can increase the risk of kidney stones. Numerous studies have contradicted each other when attempting to examine whether high doses of vitamin C lead to increased urinary oxalate levels. Two large research studies, following 45,252 men for six years and 85,557 women for 14 years, respectively, showed no increase in kidney stones in individuals who were taking 1500 mg of vitamin C or more per day when compared to those who were taking less than 250 mg of vitamin C per day. A more recent study that followed 45,619 men for 14 years did show a 41 percent increase in kidney stones in those who were taking more than 1000 mg of vitamin C per day when compared to those who were taking less than 90 mg per day. The same study showed a significant increased in the incidence of kidney stones in men who were talking only 90 to 249 mg of vitamin C per day when compared to men who took less than 90 mg per day.
High doses of vitamin C can decrease the effectiveness of blood thinners like warfarin. They can also interfere with the interpretation of certain blood tests. Let your doctor know if you are taking high doses of vitamin C.