The most common problems with excessive vitamin C are gastrointestinal: heartburn, nausea, abdominal pain, gas, and diarrhea. High doses of ascorbic acid can exert an osmotic effect in the gut and irritate sensitive individuals. Symptoms typically improve when the dose is reduced.\n\nA frequent concern is kidney stones. Part of vitamin C metabolism can increase oxalate, so people with a history of stones, susceptibility to oxalate stones, impaired kidney function, or specific risk factors should be cautious with prolonged high-dose supplementation. This does not mean everyone will develop stones, but in higher-risk groups “high dose” may represent unnecessary risk.\n\nThe concept of a tolerable upper intake level exists to keep intake within a safety margin; it is not a blanket endorsement of high-dose routines. Practical guidance: unless specifically planned by a clinician, avoid making high doses a long-term habit. The most sustainable strategy remains regular food-based intake and, if needed, modest supplementation.