In healthy adults, oral doses up to 3 grams daily of vitamin C are unlikely to cause adverse reactions. The most common adverse reaction in those who take oral doses greater than 3 grams daily are gastrointestinal and include nausea, abdominal cramps, diarrhea and flatulent distention. These reactions are attributed to the osmotic effect of unabsorbed vitamin C passing through the intestine. Some advocates of megadose vitamin C use recommend titrating the daily dose of vitamin C to what they refer to as "bowel tolerance", i.e., the point at which the user begins experiencing diarrhea. This is not recommended.
Rare adverse reactions have been reported in healthy individuals taking high oral doses of vitamin C. These include elevation of serum glucose in an adult male taking 4.5 grams daily, a gastrointestinal obstruction in a 66-year-old woman taking 4.5 grams daily of ascorbic acid and esophagitis in one person taking a single 500 milligram dose.