Many people mistakenly believe that vitamin C is not toxic and that it is beneficial to health when supplemented in large amounts.  Although vitamin C is generally low in toxicity, adverse effects have been reported when taken in very large doses.  Research indicates that gastrointestinal disturbances like nausea, abdominal cramps and diarrhea are the most common adverse effects attributable to high vitamin C intakes. Other possible adverse effects may include kidney stone formation, excess iron absorption, reduced vitamin B12 and copper levels, increased oxidative damage, and erosion of tooth enamel.  

The Tolerable Upper Intake Level (UL) is the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects in almost all healthy individuals.  The UL  for vitamin C is based on intake from diet and supplements.  There is no benefit to consuming such high amounts and the UL is not intended to be a recommended level of intake.  Different upper levels are set for different stages of the lifecycle and individuals are advised not to consume more than the UL.  The following are Tolerable Upper Intake Levels set for Vitamin C: