Cautions
Although oxalic acid is formed when ascorbic acid is metabolized, this is highly unlikely to cause renal problems in healthy individuals without pre-existing renal problems or who are not predisposed to increased crystal aggregation.
Those with pre-existing kidney stone disease or a history of renal insufficiency, defined as serum creatine greater than 2 and/or creatinine clearance less than 30, should exercise caution in the use of higher than RDA amounts of vitamin C.
Ascorbic acid is involved in modulating iron absorption and transport. It is highly unlikely that healthy individuals who take supplemental vitamin C will have any problem with iron overload. However, those with hemochromatosis, thalassemia, sideroblastic anaemia, sickle cell anaemia and erythrocyte G6PD deficiency might have such a problem if they use large amounts of vitamin C.
Drug Interactions
Aluminium-containing antacids: The intake of large doses of vitamin C used at the same time as aluminium-containing antacids has been reported to increase urinary aluminium excretion, suggesting increased aluminium absorption from these antacids. However, this is not well documented.
Aspirin: Chronic use of high dose aspirin may lead to impaired vitamin C status.
Chemotherapeutic agents: Vitamin C may potentiate the anti-neoplastic activity of cisplatin, doxorubicin and paclitaxel. It may also help ameliorate the cardio toxic effect of doxorubicin and the nephrotoxic effect of cisplatin. This is based on in vitro and animal studies. There is a concern by some researchers that supplemental doses of vitamin C may diminish the efficacy of some chemotherapeutic agents.
Estrogen: Ascorbic acid may enhance 17 beta-estradiol inhibition of oxidized LDL formation.