Vitamin C excess

What does excess vitamin C do to the digestive system?

According to the NIH, the most common complaints are diarrhea, nausea, abdominal cramps and other digestive discomfort due to the osmotic effect of unabsorbed vitamin C in the intestine. ^[1] Saturation of absorption is the background to this effect: at intakes above 1 g per day, absorption falls below 50% and the unabsorbed portion stays in the intestine. ^[1] In the hospital study by Levine and colleagues as well, bioavailability decreased at single doses of 500 mg and above, and the absorbed excess was excreted in urine. ^[6]

How many mg is the upper limit, and how is it read?

The UL is the highest daily amount not expected to cause harm when taken regularly; it is not a target or treatment amount. The US Food and Nutrition Board values cited by the NIH are below and apply to total intake from food and supplements: ^[1]

The NIH states that long-term intake above the UL may increase the risk of adverse health effects and that the UL does not apply to people taking vitamin C for medical treatment under physician supervision. ^[1]

Does vitamin C cause kidney stones?

The evidence is mixed and varies by sex, dose and study design.

• Sweden, men (Thomas et al. 2013, prospective cohort, research letter): In 1997, 48,850 men aged 45–79 were enrolled in the cohort; 23,355 men with no history of stones who used no supplements at all or used only vitamin C supplements entered the analysis (multivitamin users were examined separately). In 11 years of follow-up, 436 first kidney stone cases were identified. Ascorbic acid use was associated with a statistically significant roughly twofold increased risk (RR 1.92 in the fully adjusted model; 95% CI 1.33–2.77); RR was 1.66 (95% CI 0.99–2.79) for fewer than 7 tablets per week and 2.23 (1.28–3.88) for 7 or more tablets; multivitamin use was not associated (RR 0.86; 0.62–1.19). The authors assume the supplements contained about 1,000 mg; the dose used was not measured individually. The study covers only men, gives no information on vitamin C from food, and in an observational design confounders cannot be entirely excluded. ^[2]
• US, women and men (Ferraro et al. 2016, prospective cohort): 156,735 women in the Nurses’ Health Study I-II and 40,536 men in the Health Professionals Follow-up Study were followed (median 11.3–11.7 years, 6,245 stones). Total vitamin C intake was not significantly associated in women. In men, compared with <90 mg/day, the HR was 1.19 (0.99–1.46) for 90–249 mg, 1.15 (0.93–1.42) for 250–499 mg, 1.29 (1.04–1.60) for 500–999 mg and 1.43 (1.15–1.79) for ≥1,000 mg. For supplements alone, ≥1,000 mg/day had an HR of 1.19 (1.01–1.40) in men. Vitamin C from food was not associated in women or men; however, only a small proportion of participants had food intake >700 mg/day. The data are based on food frequency questionnaires. ^[3]
• NIH assessment: Studies examining the effect of vitamin C intake from 30 mg to 10 g/day on urinary oxalate are contradictory; the role of vitamin C in stone formation is unclear. The strongest evidence of a contribution to stone formation is in patients who already have hyperoxaluria. ^[1]

All of these studies are observational; stone risk has not been directly tested in a randomized trial. For this reason, in people with a history of stones, the decision on high-dose supplements is made with a physician.

Iron overload and hemochromatosis

Vitamin C increases the absorption of non-heme iron. The NIH reports that this is not a problem in healthy people, but that in people with hereditary hemochromatosis, chronic high-dose vitamin C can worsen iron overload and lead to tissue damage. ^[1] For intravenous administration as well, the National Cancer Institute (NCI) patient text states that high-dose IV vitamin C is not recommended for people with hemochromatosis. ^[4]

Which groups have separate warnings?

• People with kidney disease or a history of stones: The NIH states that high intake, particularly in people with kidney impairment, may contribute to stone formation by increasing urinary oxalate and urate excretion. ^[1] The NCI also writes that IV high-dose vitamin C can cause serious side effects in people with kidney disease. ^[4]
• People with G6PD deficiency: According to the NCI, high-dose vitamin C (in the context of intravenous administration) should not be given to people with G6PD deficiency, because hemolysis has been seen in case reports. ^[4]
• People receiving cancer treatment: The NIH reports that the safety and efficacy of antioxidant supplements during chemotherapy and radiotherapy are controversial; some data suggest antioxidants may protect tumor cells, other data suggest they may protect normal tissues, and some data are noted to have been criticized for design inadequacy. Whether oral supplementation changes plasma level enough to produce this effect is uncertain. ^[1]
• People using a statin-niacin combination: The NIH reports that vitamin C together with other antioxidants may reduce the HDL increase from niacin/simvastatin therapy; whether this occurs with other lipid-lowering regimens is unknown. ^[1]

The NIH also lists other reported effects of high intake (decreased B12 and copper levels, erosion of tooth enamel, allergic responses) and states that at least some of these arose from measurement-method artifacts and were not confirmed in later studies. In the Iowa Women’s Health Study, in a subgroup of postmenopausal women with diabetes, supplements of ≥300 mg/day (not intake from food) were associated with cardiovascular death; the NIH notes that this is a subgroup finding and has not been seen in any other epidemiological study. ^[1]

Does high-dose vitamin C affect laboratory measurements?

In a study with hospital-type strip-based blood glucose meters (Katzman et al. 2021), vitamin C concentrations from standard doses up to the very high doses used in oncology protocols were tested. In two of the three devices, increasing ascorbic acid concentration led to falsely high glucose values; the third device gave no false values but, above a certain threshold, gave an error message and produced no result. The context of the study is an oncology patient receiving intravenous high-dose therapy. ^[5] The scale of this effect with oral supplements is not the subject of this study.

Whether new complaints arising during high-dose supplement use are due to vitamin C cannot be determined from symptoms alone; assessment is made by a physician together with information on the product and amount used.

This guide does not recommend personal doses.