What happens if you take too much vitamin C?

Stomach and gut effects

The most common problems with high doses are diarrhea, nausea and stomach cramps.^[1] They happen because the vitamin C that the gut does not absorb draws water into the gut.^[1] The gut absorbs less than half of doses above 1 g a day.^[1]

Upper limits

The US upper limits are for food and supplements together:^[1]

• 1 to 3 years: 400 mg a day^[1]
• 4 to 8 years: 650 mg a day^[1]
• 9 to 13 years: 1,200 mg a day^[1]
• 14 to 18 years: 1,800 mg a day, also in pregnancy and breastfeeding^[1]
• adults: 2,000 mg a day, also in pregnancy and breastfeeding^[1]
• infants: no limit set; they should get vitamin C only from breast milk, formula and food^[1]

In the EU, EFSA did not set an upper limit because the data were not enough.^[2] It considered that 1,000 mg a day on top of the usual diet is not linked to gut side effects.^[2] The German Federal Institute for Risk Assessment (BfR) proposes at most 250 mg of vitamin C in the daily dose of a supplement.^[2]

Kidney stones

In 40,536 US men followed for about 11 years, supplements of 1,000 mg a day or more went with a 19% higher risk of kidney stones.^[3] In 156,735 women, there was no such link.^[3] A 2019 review of 4 studies found the same pattern.^[4] In a hospital study, volunteers passed more oxalate and urate in urine at 1,000 mg a day than at lower doses.^[5] Both could add to the formation of kidney stones, but studies of vitamin C and oxalate in urine gave conflicting results.^[1] The clearest risk is in people who already have high oxalate in their urine.^[1]

Iron overload

Vitamin C helps the gut absorb iron from plant foods.^[1] In healthy people this does not cause too much iron.^[1] In people with hereditary hemochromatosis, an inherited iron overload, long use of high doses could add to the iron and damage tissues.^[1]

Other concerns

Some reports linked high doses to lower vitamin B12 and copper levels, damage to tooth enamel and allergies.^[1] At least some of these were errors in the lab tests, and later studies did not confirm them.^[1]

Medicines

• Chemotherapy and radiation therapy. It is not clear whether antioxidants protect tumor cells or healthy cells from these treatments. People in cancer treatment should ask their oncologist first.^[1]
• A statin with niacin. Vitamin C with other antioxidants may reduce the rise in HDL cholesterol that this combination gives.^[1]

Who should ask first

Ask a doctor or pharmacist before you take more than the upper limit, or any high dose if you have kidney disease, kidney stones or hemochromatosis.^[1] The upper limits do not apply to people who get vitamin C as a medical treatment under a doctor's care.^[1]