Too much vitamin C is unlikely to cause serious harm, but it can make you pretty uncomfortable. The most common result of overdoing it is digestive distress: diarrhea, nausea, and abdominal cramps. Your body has a limited ability to absorb this vitamin, so once you exceed what it can use, the excess either passes through your gut (causing those symptoms) or gets flushed out through urine.
How Much Is Too Much
Adults need about 75 to 90 mg of vitamin C per day, with smokers needing an extra 35 mg. The tolerable upper intake level for adults is 2,000 mg per day. That’s not a target or a recommendation. It’s the ceiling above which side effects become increasingly likely. Children have lower thresholds depending on age.
For context, a single orange contains roughly 70 mg of vitamin C. You’d need to eat dozens of oranges to approach that 2,000 mg ceiling. In practice, excess vitamin C almost always comes from supplements, not food.
Why Your Body Can’t Store the Excess
Your intestines absorb 100% of vitamin C when you take doses up to about 200 mg at a time. Above 500 mg, absorption drops sharply. The higher the dose, the smaller the fraction your body actually takes in.
Whatever does get absorbed but isn’t needed gets filtered out by your kidneys. Doses above 400 mg per day are almost entirely excreted in urine. In one study, people taking 2,000 mg supplements had similar levels of vitamin C in their urine as people taking just 100 mg, because the body was dumping the surplus either way. This is why megadoses don’t meaningfully raise your blood levels of the vitamin. Your body simply has no interest in hoarding it.
Digestive Symptoms Are the Main Problem
The vitamin C that your intestines can’t absorb stays in your digestive tract, where it draws water into the gut through osmosis. This is the same mechanism behind certain laxatives. The result is watery diarrhea, cramping, bloating, and nausea. These symptoms are dose-dependent: the more you take beyond your absorption limit, the worse they get. They typically resolve once you cut back.
This is the most common complaint by far, and for most healthy people, it’s the only real consequence of taking too much.
Kidney Stone Risk
Your body converts some excess vitamin C into oxalate before excreting it. Oxalate is one of the main components of kidney stones. High-dose supplementation increases the amount of oxalate in your urine, which raises the theoretical risk of calcium oxalate stones. If you’ve had kidney stones before or have a history of kidney disease, large vitamin C doses are worth avoiding for this reason alone.
Iron Absorption and Hemochromatosis
Vitamin C enhances iron absorption from food, which is helpful for most people but potentially dangerous for those with hemochromatosis, a genetic condition that causes the body to store too much iron. For healthy individuals, even high vitamin C intakes don’t cause iron imbalance. People who are carriers of the hemochromatosis gene (heterozygous) are also likely fine. But people with full-blown hemochromatosis should limit anything that increases iron absorption, including high-dose vitamin C supplements.
Interference With Medical Tests
One lesser-known consequence of very high vitamin C levels is that they can throw off certain lab results. Glucose meters, in particular, are vulnerable. High concentrations of vitamin C in the blood can cause falsely elevated blood sugar readings on some devices. In testing, two out of three common hospital glucose meters gave inaccurately high readings when vitamin C was present, which could lead to incorrect treatment decisions. A third meter detected the interference and simply refused to display a result.
If you’re taking high-dose vitamin C for any reason and need bloodwork or glucose monitoring, let your healthcare provider know. The interference can affect test accuracy in ways that matter for your care.
The Rebound Effect
If you’ve been taking very high doses of vitamin C for an extended period, stopping abruptly can cause your blood levels to drop below where they were before you started supplementing. This is called the rebound effect. Your body adjusts to processing large amounts by ramping up its excretion machinery. When the supply suddenly disappears, that enhanced excretion continues for a time, potentially draining your stores faster than normal.
In extreme cases, this could produce symptoms resembling early scurvy: fatigue, irritability, and gum problems. The practical takeaway is that if you’ve been on megadoses, taper down gradually rather than quitting cold turkey. This gives your body time to recalibrate.
What a Sensible Intake Looks Like
Most people get enough vitamin C from a diet that includes fruits and vegetables. A serving of bell peppers, strawberries, broccoli, or citrus fruit easily covers your daily needs. Supplements in the 100 to 200 mg range are fully absorbed and well tolerated if you want extra insurance.
Going above 1,000 mg per day offers diminishing returns because your body simply excretes what it can’t use. The digestive side effects, kidney stone risk, and test interference all start becoming relevant concerns in that range, with essentially no proven benefit for healthy people. If you’re experiencing stomach upset from a supplement, the simplest fix is to take less of it.