Very large doses of vitamin C can cause gastric irritation, flatulence, or diarrhea. Oxalate kidney stones are of theoretical concern because ascorbic acid is metabolized to oxalate, but stone formation has not been frequently reported. Vitamin C can also confound common diagnostic tests by causing false-negative results for some fecal occult blood tests and both false-negative and false-positive results for urine glucose.
et al. Dietary vitamin C in human health. Adv Food Nutr Res. 2018;83:281.
et al. Plasma concentrations of ascorbic acid in a cross section of the German population. J Int Med Res. 2018;46:168.