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Vitamin C is a water-soluble substance, and therefore does not accumulate in the body. This accounts for the absence of any data on vitamin C toxicity per se. Vitamin C in itself is of low toxicity, and has only minor adverse effects, such as diarrhea, nausea and other digestive disturbances.
These upsets are due to the osmotic withdrawal of water from the intestinal contents by the unabsorbed vitamin C in the gastrointestinal tract. Other physiologic disruptions due to high vitamin C levels include metabolic acidosis and altered levels of some coagulation factors such as prothrombin.
Vitamin C metabolism produces oxalate as a product. Some research suggests that vitamin C supplements may increase urinary oxalate concentrations, and double the risk of calcium oxalate kidney stones.
Several drugs reduce vitamin C levels, such as the contraceptive pills that contain estrogen, barbiturates and tetracyclines. Vitamin C also raises estrogen levels.
Regular aspirin intake above a certain frequency, such as 1000 mg every 6 hours, increases the urinary excretion of vitamin C and reduces its blood levels. Vitamin C causes higher serum levels of aspirin and other NSAIDs.
Vitamin C can inhibit the action of warfarin, and thus the dosage of warfarin requires to be monitored by the prothrombin time. It may also reduce the cardioprotective action of cholesterol-lowering drugs such as simvastatin and niacin, though the evidence is far from clear. Some large trials indicate that it does not affect the positive effects of these drugs.
It has been suggested that vitamin C use during cancer cell chemotherapy could diminish the efficacy of treatment by protecting tumor cells from destruction. Other studies, though, indicate that vitamin C exerts a possible protective effect on normal cells against the damage induced by radiotherapy or chemotherapy.
It has also been suggested that high doses of vitamin C could lead to oxidative damage by contributing to free radical production, but this has not been experimentally shown.
Tests such as serum bilirubin, serum creatinine and the guaiac test for the presence of occult blood depend upon the presence of reducing substances, and thus high levels of vitamin C can interfere with their interpretation.
A single study showed that vitamin C supplementation appeared to increase the cardiovascular disease mortality in post-menopausal diabetic women, but this has never been confirmed.
Vitamin C may theoretically cause excessive iron absorption, but this is likely to be of significance only in those who have hereditary hemochromatosis and consume high doses of vitamin C.
When taken along with aluminium-containing antacids, vitamin C boosts aluminium absorption and therefore the risks of side-effects.
Megadoses of vitamin C in pregnancy may cause rebound scurvy in the infant, due to the sudden drop in levels causing a relative deficiency of the vitamin in the baby’s blood after birth. Thus, doses above 1000 mg vitamin C should be avoided in pregnancy unless on medical advice.
Other dubious effects, which have been raised over the years, but never been proven to exist, include:
FNB-established upper levels are age-specific, and are intended to prevent side effects. Precautions to prevent adverse reactions include: