Can you get too much vitamin C?

The Tolerable Upper Intake Level for vitamin C is 2,000 mg a day for adults.

What this means for you

The short answer is that food will not do it to you. Two thousand milligrams is more than twenty oranges in a day. The people who reach that number are taking tablets, usually 1,000 mg at a time, usually because a cold is starting.

What happens first is dull rather than dangerous. Digestive upset, which means loose stools, nausea and cramping, and it goes away when the dose does. The body stops absorbing vitamin C efficiently at high intakes and clears the rest through the kidneys, which is why the megadose you paid for mostly ends up in the toilet.

Not for everyone. There are two situations where high doses deserve more care than that. High vitamin C intakes can raise oxalate and uric acid in urine, which may contribute to kidney stones, so if you have formed one before, a big daily dose is not a neutral habit. And if you are having chemotherapy or radiation, ask your oncologist before taking vitamin C supplements, especially large ones.

Kidney stones, and why the answer splits by sex

The stone question used to be a maybe on this page. It now has numbers behind it, and they do not fall the same way for everyone.

Not for everyone. Pooling four studies of people taking vitamin C supplements, men had about 60 per cent higher odds of a kidney stone than women on the same habit. Among men the raised risk showed up at 250 to 499 milligrams a day and again at 1,000 to 1,500, both by around 12 to 14 per cent. The bands in between did not reach significance, and that gap is a warning about the data rather than a safe window inside it. Four observational studies cannot tell you where your own line sits.

In women the same review found nothing at any dose it looked at, from 250 milligrams up to more than 1,500. That is a null across a group of women. It is not a clearance for a woman who has already passed a stone.

What happens in the body has been watched directly. On a controlled diet, 48 people took a gram of vitamin C twice a day for six days. In 19 of them, four out of ten, the oxalate leaving in the urine rose by more than a tenth, and the calculated stone risk went from 0.76 to 1.10. People who had formed stones before and people who never had responded in much the same proportions. Six days, urine chemistry rather than actual stones, and two grams a day is exactly the stated ceiling. Six in ten did not respond at all, and nothing tells you in advance which group you are in.

The drip is a different drug

Vitamin C given into a vein in intensive care is nothing anyone reaches by swallowing tablets. It belongs on this page because it is the one setting where vitamin C has been shown to make people worse.

Not for everyone. In 872 adults in intensive care with infection, 44.5 per cent of those given intravenous vitamin C were dead or still on organ support at 28 days, against 38.5 per cent on placebo. The risk ratio is 1.21 and the interval does not cross one. One severe low-blood-sugar episode and one anaphylaxis happened in the vitamin C group and none in the other.

Unsettled. The pooled work that trial contradicts is worth seeing for what it was. Across 18 trials in 3,364 patients, intravenous vitamin C improved an organ-failure score by 0.62 points and shortened time on blood-pressure drugs by about 15 hours, while deaths did not move at all. Adverse events roughly doubled. A 0.62-point score and fifteen hours are things a chart notices, and whether a patient notices them was never shown. The survival benefit people quoted from that pool came from one dose subgroup rather than from the comparison the trials were built to make.

For everyone else the practical line is simple. A supplement of a few hundred milligrams with food does everything a gram does, and the fruit bowl is not something you need to ration. Five varied servings of fruit and vegetables a day already supply more than 200 mg, which is twice the requirement and nowhere near the 2,000 mg ceiling.

Worth remembering why people reach the gram-a-day territory in the first place. Taking vitamin C regularly does not reduce the chance of catching a cold in the general population, and it shortens one slightly, by about 8 per cent in adults and 14 per cent in children. For a week-long cold that is roughly half a day. The megadose is being taken for an effect that was measured and turned out to be small.

The gram-a-day habit deserves one more number. In the Cochrane review, starting vitamin C after symptoms began had no consistent effect on how long a cold lasted or how bad it was, across seven comparisons and 3,249 cold episodes. The effect that does exist belongs to people who were already taking it daily. Taking more once you feel ill is the version that buys nothing and costs you the digestive upset. the review of both is here.

Covid gave that question a bigger test than sepsis had. Across nine randomised trials in 3,345 patients, vitamin C given into a vein in all but one of them, deaths came out no different from standard treatment. The risk ratio was 0.92 with an interval from 0.83 to 1.02, and the trials agreed with each other almost perfectly. In the critically ill it was 0.90. In everyone else 1.03.

Myth. Intravenous vitamin C did not save lives in covid, at any level of illness that was studied. Three thousand patients in randomised trials is about as clear as a null result gets, and it is the same answer the sepsis trials gave.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

Upper limits apply to everything you take in, from supplements and, for most nutrients, from food as well. Reaching one from whole food alone is uncommon. Supplements are where people cross the line, usually without meaning to.

Full guide: Vitamin C: what it does, how much you need, and what it will not do