A megadose of vitamin C is any amount that far exceeds the recommended daily intake of 75 to 90 mg for adults, typically starting at 1,000 mg (1 gram) and ranging up to 10 grams or more per day. The concept was popularized in the 1970s by Nobel laureate Linus Pauling, who suggested that gram-level doses could prevent and treat everything from the common cold to cancer. Today, megadosing remains popular in supplement culture, but the science behind it is more nuanced than the enthusiasm suggests.
How Much Is a Megadose?
The official recommended daily allowance for vitamin C is 90 mg for adult men and 75 mg for women, with smokers needing an extra 35 mg per day. The tolerable upper intake level, the highest amount considered unlikely to cause harm, is set at 2,000 mg per day for all adults. Most people use the term “megadose” to describe anything at or above 1,000 mg, though some proponents advocate for 3 to 10 grams daily, and certain alternative health protocols push even higher.
Linus Pauling initially recommended around 2,000 mg per day in the early 1970s, then raised his suggestion significantly in his 1986 book. His recommendations were based on theoretical arguments: cross-species comparisons, evolutionary reasoning, and estimates of how much vitamin C early humans consumed from raw plant foods. He did not have the pharmacokinetic data available today.
Your Body Has an Absorption Ceiling
One of the most important things to understand about vitamin C megadoses is that your body can only absorb so much at once. Vitamin C relies on active transport in the intestines, and that system saturates at relatively modest doses. Research on blood levels shows that plasma and circulating cells become fully saturated at intakes of about 200 to 400 mg per day in healthy people. Pauling himself acknowledged this in a 1974 interview, noting that “the first 250 mg is more important than any later 250 mg” because it brings blood levels to saturation.
Once your tissues are saturated, extra vitamin C is simply filtered out by your kidneys. So if you take 2,000 mg in a single oral dose, you absorb a much smaller fraction than if you took 200 mg. The unabsorbed portion passes into the colon, which is where side effects come from.
Side Effects of High Doses
Doses up to 2,000 mg per day cause virtually no side effects in most people. Above that threshold, the unabsorbed vitamin C draws water into the intestines through osmosis, which commonly causes diarrhea, nausea, abdominal cramps, and bloating. Some people also experience flushing, dizziness, and headaches. Single large doses are more likely to trigger these symptoms than the same amount spread throughout the day.
The more serious concern is kidney stones. Your body converts excess vitamin C into oxalate, and doses above 1,000 mg per day can increase oxalate excretion by 6 to 13 mg per day. That extra oxalate can combine with calcium in the kidneys to form calcium oxalate stones, the most common type. People with preexisting kidney problems are at particular risk. Oxalate-related kidney damage has been reported after surprisingly short courses of high-dose vitamin C in patients with compromised kidney function.
Iron Absorption
Vitamin C enhances absorption of non-heme iron, the type found in plant foods and supplements. For people with low iron stores, this is a benefit. But for people with hereditary hemochromatosis (a genetic condition causing iron overload), high-dose vitamin C could theoretically worsen the problem. Research has found that healthy people and even carriers of one hemochromatosis gene tolerate high vitamin C without iron imbalance, but the effect on people with full-blown iron overload hasn’t been well studied.
What the Cold and Flu Evidence Actually Shows
The most common reason people megadose vitamin C is to fight off colds, and the evidence here is real but modest. Randomized trials consistently show that regular vitamin C supplementation shortens the duration of colds, though it doesn’t reliably prevent them in the general population.
The more interesting finding is that vitamin C’s effect is concentrated on severe symptoms rather than mild ones. In one of the largest cold trials ever conducted, covering over 1,170 illness episodes, vitamin C reduced days “confined to house” by 21% per episode. The overall reduction in housebound days per person was 30%. A meta-analysis found a statistically significant difference between vitamin C’s effect on severe versus mild cold symptoms: it meaningfully shortened severe symptoms but had no significant effect on mild ones. Some trial data suggest that 6 to 8 grams per day might be roughly twice as effective as 3 to 4 grams, but this comes from a limited number of comparisons.
In practical terms, this means megadosing vitamin C during a cold may trim a day or so off the worst part of your illness. It’s not a cure, but for people who get frequent or severe colds, the effect on the hardest-hit days could be worthwhile.
IV Vitamin C in Hospitals
Intravenous vitamin C bypasses the intestinal absorption limit entirely, allowing blood levels to reach concentrations 100 times higher than any oral dose could achieve. This has made it a subject of interest in critical care, particularly for sepsis (a life-threatening response to infection that kills an estimated 11 million people worldwide each year).
The results have been disappointing. The largest and most rigorous trial, known as LOVIT, randomized 872 ICU patients with sepsis to receive either intravenous vitamin C or a placebo for up to 96 hours. The vitamin C group actually fared worse: 44.5% experienced death or persistent organ dysfunction at 28 days, compared to 38.5% in the placebo group. However, subsequent analysis raised the possibility that the harm wasn’t from the vitamin C itself but from abruptly stopping it. Earlier trials with the same dosing protocol showed that vitamin C appeared beneficial during the four-day infusion, with the benefit sharply disappearing after treatment stopped.
The Rebound Effect
When you take high doses of vitamin C for a period and then suddenly stop, your blood levels can temporarily drop below where they were before you started supplementing. This is called the rebound effect, and it has been documented in multiple studies. The body appears to ramp up its vitamin C metabolism during high-dose supplementation, so when the supply vanishes, it burns through remaining stores faster than normal.
In critically ill patients, this rebound can be dangerous. Cases of scurvy (the disease caused by severe vitamin C deficiency) have been reported in Western hospitals, and a few critically ill patients have died from it. For otherwise healthy people, the rebound is unlikely to cause scurvy, but it’s a good reason to taper off gradually rather than stopping a high-dose regimen all at once. If you’ve been taking several grams a day, stepping down over a week or two gives your body time to readjust.
What a Practical Approach Looks Like
Given the absorption data, the sweet spot for most healthy people is 200 to 400 mg per day. That’s enough to fully saturate your blood and tissues, and it’s easily achievable through diet: a single orange and a cup of broccoli get you there. If you want to supplement on top of that, doses up to 2,000 mg per day are considered safe for the vast majority of adults, though much of it will simply be excreted.
If you’re fighting a cold and want to try higher doses for a few days, the trial evidence suggests that several grams per day may shorten the worst symptoms. Splitting the dose across the day improves absorption and reduces the chance of digestive upset. People with kidney disease, a history of kidney stones, or iron-overload conditions should be cautious with any dose above the RDA.