High-Dose Vitamin C: Kidney Stones, Diarrhea, Lab Interference and Upper Limits

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Vitamin C is water-soluble, and MedlinePlus says serious side effects from too much are very rare because the body cannot store it. [1] That does not make large amounts harmless for everyone. People with kidney stones or kidney disease, iron overload, G6PD deficiency or diabetes who check glucose, and people in pregnancy or cancer treatment, should talk to a clinician or pharmacist before using high-dose vitamin C. If a child has swallowed a large number of vitamin tablets or gummies, see Poisoning: what to do first and Poison help by country. In the US, Poison Help is 1-800-222-1222.

This page is information only. It does not give personal doses.

Evidence labels used here: evidence of harm, mixed, preclinical, insufficient evidence and Proper evidence not found.

Where "too much" starts depends on who you ask

An upper level is a ceiling for the general population, not a target and not a recommendation. See RDA, AI, DV and UL. EFSA also noted that absorption of vitamin C is saturated at high doses, so intakes above 1 g a day add little uptake but raise the risk of digestive side effects, and that long-term safety of high-dose supplements has not been systematically assessed. [4] These are four official bodies with different approaches, not a contradiction to resolve.

Stomach upset and diarrhea

Diarrhea, stomach pain and gas are the best-documented effects at high intakes. [2][4] The NHS says these should disappear once you stop taking the supplement. [2]

Kidney stones

Observational studies show association, not proof of cause, and people who take supplements can differ in other ways. Urinary oxalate is a lab measure, not a stone. Anyone with a history of stones should ask their clinician before using a high-dose supplement.

Kidney disease

High-dose intravenous vitamin C has been followed by renal failure in patients with existing kidney disorders, according to the National Cancer Institute's summary. [10] For oral use in kidney disease, Proper evidence not found in the sources we read, so general rules do not apply: ask your kidney team. A 2026 systematic review of people on hemodialysis found that vitamin C status can be low, and said current data does not justify routine use for conditions such as chronic inflammation. [11] See Vitamins and kidney or liver disease and Minerals, supplements and kidney disease.

Iron overload

People with haemochromatosis are advised by the NHS to avoid iron and vitamin C supplements, which may be harmful when iron levels are high. [12] See Vitamin C and iron absorption.

G6PD deficiency

The National Cancer Institute's summary says case reports indicate people with G6PD deficiency should not receive high doses of vitamin C because of the risk of hemolysis (breakdown of red blood cells). [10] A review of 14 case reports found 71.4% of patients had G6PD deficiency, doses ranged from 1 to 200 g a day, and 57.1% received intravenous formulations. [13] These reports mostly involve very large intravenous amounts. Whether typical oral supplements carry the same risk: Proper evidence not found in the sources we read. Anyone who knows they have G6PD deficiency should ask their clinician.

Glucose meters and other lab tests

High-dose intravenous vitamin C has been shown to interfere with lab measurements, including strip-based glucose meters, which can show falsely high readings. [10] In a laboratory study of seven commercial meters, several showed a positive bias in blood samples spiked with vitamin C. [14] A 2026 laboratory study spiked blood samples with ascorbic acid and found severe positive interference on one hospital meter (up to 98.8%) but minimal interference on a newer one (about -4.4% at most). [15] Meters differ. Whether ordinary oral supplements raise blood levels enough to affect readings: Proper evidence not found in the sources we read. If you check your glucose or have blood tests, tell the care team about any high-dose vitamin C so they can interpret results.

Pregnancy, children, older adults and surgery

MedlinePlus says large supplemental doses are not recommended in pregnancy, because they can lead to a shortage in the baby after delivery. [1] See Pregnancy and breastfeeding safety and Vitamins for children and teens. Tell the surgical team about supplements: Supplements before surgery. Intravenous use in cancer is covered in Intravenous vitamin C and cancer.

Quality and labels

NCI notes that ingredients in dietary supplements may vary from lot to lot. [10] Look at the mg per serving and number of servings, and see Quality testing and certifications and Reading supplement labels. Ask a pharmacist about medication interactions.