Vitamin C Toxicity (Ascorbic Acid Toxicity) By Larry E Johnson, MD, PhD, University of Arkansas for Medical Sciences Reviewed / Revised Nov 2022 View Patient Education Intake of > 2 g of vitamin C a day may cause nausea and diarrhea, interfere with the healthy antioxidant-prooxidant balance in the body, and, in patients with thalassemia or hemochromatosis, promote iron overload.