Vitamin C

Is vitamin C safe?

Yes, for nearly everyone at the doses on supermarket shelves. Your body absorbs less vitamin C as the dose rises and passes the rest out in urine, so it is hard to build up a toxic amount. The US upper limit of 2,000 mg a day was set to prevent diarrhoea and stomach cramps, not organ damage, and the European Food Safety Authority (EFSA) decided in 2004 that there was not enough evidence of harm to set a limit at all.

The exception worth knowing about is kidney stones. Some of the vitamin C you take is turned into oxalate, the main ingredient of the most common type of stone, and men who take high-dose tablets every day appear to get more of them.

How much vitamin C do you actually need?

Far less than most supplements contain. The NHS says adults need 40 mg a day and can get it from diet; the US recommended intake is 75 mg for women and 90 mg for men, plus 35 mg for smokers. One orange or a handful of strawberries covers it. The Linus Pauling Institute, which takes a more generous view, suggests 400 mg a day, still well below the 1,000 mg tablets that dominate the market.

Age	US upper limit (mg a day)
1 to 3	400
4 to 8	650
9 to 13	1,200
14 to 18	1,800
Adults	2,000

Above about 1,000 to 2,000 mg, unabsorbed vitamin C draws water into the gut. The NHS lists stomach pain, diarrhoea and flatulence as the effects of taking more than 1,000 mg a day. These go away when you cut the dose.

Does vitamin C cause kidney stones?

The evidence is mixed, but it points one way for men. In the US Health Professionals Follow-Up Study, which tracked 45,619 men for 14 years, those getting 1,000 mg or more of vitamin C a day had a 41% higher risk of kidney stones than men getting under 90 mg. A Swedish study of 48,840 men followed for 11 years, published in JAMA Internal Medicine in 2013, found that regular users of single vitamin C supplements had about twice the risk of stones. Two other large cohorts, including one of 85,557 women, found no increase.

The more serious but much rarer problem is oxalate nephropathy, where oxalate crystals clog the kidney itself. A 2014 case in the Clinical Kidney Journal described a woman who took more than 4 g a day for a month as an alternative cancer treatment; her kidneys did not recover after three weeks of dialysis. Most reported cases involve several grams a day, intravenous vitamin C, existing kidney disease, or gut conditions that raise oxalate absorption.

Interactions with medicines

Vitamin C has few interactions, and most only matter at high doses:

• Warfarin: the evidence is weak and contested, but large doses may reduce its effect. The Linus Pauling Institute suggests people on anticoagulants keep vitamin C under 1,000 mg a day and have their INR checked if they change their intake.
• Chemotherapy and radiotherapy: antioxidants might protect tumour cells from some treatments. Anyone having cancer treatment should ask their oncologist before taking vitamin C.
• Aluminium-containing antacids and phosphate binders: vitamin C increases aluminium absorption, a risk for people with poor kidney function.
• Oestrogen: supplements may raise blood oestrogen in women on the pill or HRT.
• Lab tests: high doses can distort some blood and stool test results, so tell whoever orders the test.

Vitamin C also increases iron absorption. That is useful if you take iron for a deficiency, and harmful for people with haemochromatosis, whose bodies already store too much iron.

Who should be careful

• People who have had kidney stones, especially calcium oxalate stones.
• Anyone with chronic kidney disease or on dialysis.
• People with haemochromatosis or thalassaemia.
• People having chemotherapy or radiotherapy.
• People taking warfarin who plan to take more than 1,000 mg a day.

If any of these apply, talk to your doctor or pharmacist before taking a supplement. For everyone else, the practical advice is to take less than you think. A 200 to 500 mg tablet, or simply more fruit and vegetables, gets your blood levels close to their ceiling; 1,000 mg effervescent tablets mostly add cost and trips to the toilet.

How we scored it

Regulatory record (92) and dose safety (90) are the strongest factors: no regulator has restricted vitamin C, and the upper limit exists to prevent stomach upset. Interactions (86) and quality (85) are also good. Organ toxicity (80) is the weakest because of the kidney stone link in men and rare oxalate kidney failure from megadoses. The score sits just above vitamin D, which builds up in the body and causes harm at lower multiples of its upper limit.