Can too much vitamin C make you feel bad? Understanding the risks of megadosing

October 7, 2025 •

4 min read

The tolerable upper intake level (UL) for vitamin C in adults is 2,000 mg per day, with doses exceeding this threshold often causing unpleasant side effects. So, can too much vitamin C make you feel bad? The short answer is yes, particularly when consuming large amounts from supplements, leading to a range of discomforts and potential health risks.

Quick Summary

High-dose vitamin C supplements, especially those exceeding the 2,000 mg daily limit, can trigger side effects like nausea, diarrhea, and stomach cramps. This is often due to the body's inability to process the excessive amount, and it can also increase the risk of more serious issues like kidney stones in some individuals.

Key Points

Gastrointestinal Distress: The most common effect of too much vitamin C is digestive upset, including diarrhea, nausea, and stomach cramps, caused by unabsorbed ascorbic acid.
Tolerable Upper Intake Level: The maximum safe daily dose for adults is 2,000 mg; exceeding this limit increases the risk of adverse effects.
Kidney Stone Risk: High-dose vitamin C supplements can increase urinary oxalate, which raises the risk of forming calcium oxalate kidney stones in susceptible individuals.
Food vs. Supplements: It is nearly impossible to get excessive vitamin C from food alone, making supplements the primary source of problematic megadoses.
Specific Health Concerns: Individuals with conditions like hemochromatosis (iron overload) or G6PD deficiency should avoid high-dose vitamin C, as it can worsen their conditions.
Absorption Decreases with Dose: The body's ability to absorb vitamin C declines significantly at high doses, meaning a large portion is wasted and can cause negative side effects.

The Water-Soluble Myth: Why More Isn't Always Better

It's a common misconception that because vitamin C is a water-soluble vitamin, any excess is harmlessly flushed from the body. While it's true that the body excretes what it doesn't need through urine, this isn't an instantaneous or completely benign process. When a person ingests a large, concentrated dose—far beyond what the body can absorb at once—the unabsorbed vitamin C, or ascorbic acid, can cause significant distress within the gastrointestinal tract. Absorption rates decrease significantly when intakes exceed 1,000 mg per day, meaning a large portion of a megadose will remain in the gut and cause irritation. The resulting osmotic effect draws water into the intestines, triggering the very symptoms that can make you feel unwell.

Common Gastrointestinal Side Effects

One of the most immediate and common signs of excessive vitamin C intake is gastrointestinal distress. The severity often depends on the dosage and individual sensitivity, but typical symptoms include:

Diarrhea: This is a direct consequence of the osmotic effect caused by unabsorbed vitamin C in the intestines. It is often one of the first indicators of a megadose.
Nausea and Vomiting: The acidity of large amounts of ascorbic acid can irritate the stomach lining, leading to feelings of nausea and, in some cases, vomiting.
Abdominal Cramps and Bloating: Stomach discomfort, including cramping and general feelings of fullness, is frequently reported with high vitamin C consumption.
Heartburn: For some individuals, the acidic nature of vitamin C can exacerbate acid reflux symptoms and cause heartburn.

The Link to Kidney Stones

While the gastrointestinal effects are temporary, a more serious risk associated with high-dose vitamin C supplementation is the formation of kidney stones. Vitamin C is metabolized into oxalate, a waste product excreted in urine. When urinary oxalate levels are high, it can combine with calcium to form calcium oxalate crystals, which are the most common component of kidney stones. Research has shown a correlation between high-dose vitamin C supplements and an increased risk of kidney stones, particularly in men. This risk is especially relevant for those with a prior history of kidney stones or chronic kidney disease.

The Tolerable Upper Intake Level (UL)

For most healthy adults, the Tolerable Upper Intake Level (UL) for vitamin C is 2,000 mg per day. This represents the maximum daily intake that is unlikely to cause harmful side effects. While short-term consumption slightly above this level may cause only temporary stomach upset, consistent megadosing can pose more significant health risks over time. It is important to remember that this UL applies to intake from all sources, including food and supplements.

How to Assess Your Vitamin C Intake

Before taking any supplement, consider your dietary intake. The average adult can easily meet their daily vitamin C requirements through a balanced diet rich in fruits and vegetables. For example, a single orange or serving of broccoli can provide a significant portion of the recommended daily amount of 75-90 mg for adults. Tracking your total intake is crucial to ensure you don't inadvertently exceed the UL through a combination of fortified foods, multivitamin supplements, and additional vitamin C products.

Health Conditions Affected by High Vitamin C Intake

Certain individuals face specific health risks when taking high doses of vitamin C. Anyone with a pre-existing medical condition should consult a healthcare provider before taking supplements.

Hemochromatosis and Iron Overload

Vitamin C significantly enhances the absorption of nonheme iron, the type found in plant-based foods. For individuals with hemochromatosis, a genetic disorder causing excessive iron accumulation in the body, high-dose vitamin C can worsen iron overload and lead to severe tissue damage.

G6PD Deficiency and Hemolysis

People with Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency, a metabolic condition, can experience hemolysis (the destruction of red blood cells) from large doses of vitamin C.

Food Sources vs. Supplement Sources

Getting vitamin C from natural food sources is far safer than from supplements, as it is nearly impossible to consume a megadose from diet alone. Food-based vitamin C comes packaged with a host of other beneficial nutrients that aid overall health. For an excellent resource on dietary sources, consider exploring the Harvard T.H. Chan School of Public Health's information on vitamin C available online. Supplementation, in contrast, delivers a concentrated, isolated dose that the body may struggle to process, increasing the likelihood of side effects.

Comparison Table: Food vs. High-Dose Supplements


Feature	Food-Based Vitamin C	High-Dose Supplements (>1,000 mg)
Absorption	Highly efficient, absorbed along with other nutrients.	Decreased absorption rate; less than 50% absorbed at high doses.
Gastrointestinal Distress	Very rare, as dose is naturally spread throughout the day.	Common, including diarrhea, nausea, and cramping.
Kidney Stone Risk	Minimal to no increased risk, even with high dietary intake.	Increased risk, especially for those predisposed.
Associated Nutrients	Includes a full spectrum of vitamins, minerals, and fiber.	Isolated nutrient; lacks the holistic benefits of whole foods.
Interaction Risks	Minimal.	Potential to interfere with medications and exacerbate certain conditions.

A List of Vitamin C-Rich Foods

Citrus Fruits: Oranges, grapefruits, and lemons.
Bell Peppers: Red and green varieties are excellent sources.
Berries: Strawberries, blueberries, and raspberries.
Cruciferous Vegetables: Broccoli, cauliflower, and Brussels sprouts.
Tropical Fruits: Kiwi, papaya, and mango.

Conclusion: Moderation is Key

While vitamin C is an essential nutrient for many bodily functions, including immune health and collagen production, excessive intake is not without risks. The idea that megadosing on vitamin C is a harmless, or even beneficial, practice has been largely debunked. Most individuals can obtain more than enough vitamin C through a balanced diet, and supplement use, especially at high doses, should be approached with caution. By staying within the tolerable upper intake level of 2,000 mg per day for adults, you can harness the benefits of vitamin C while avoiding the unpleasant and potentially harmful side effects that can make you feel bad.

Frequently Asked Questions

The Tolerable Upper Intake Level (UL) for vitamin C in healthy adults is 2,000 mg per day. Doses exceeding this amount increase the risk of side effects like digestive issues.

Symptoms of excessive intake typically include gastrointestinal issues like diarrhea, stomach cramps, nausea, and heartburn. These are often the first signs that you have consumed more than your body can handle.

Yes, high-dose vitamin C can increase the amount of oxalate in your urine, which can contribute to the formation of calcium oxalate kidney stones, especially in people with a history of them.

High-dose supplements are the primary cause of negative side effects, as it is very difficult to consume enough vitamin C from food to cause problems. Food-based vitamin C is absorbed more efficiently and poses minimal risk.

Individuals with hemochromatosis, G6PD deficiency, or a history of kidney stones should be particularly cautious with vitamin C supplements and consult a doctor.

Yes, high doses of vitamin C can affect the absorption of other substances. For example, it significantly increases the absorption of nonheme iron.

While rare, some anecdotal reports and older theories suggest that suddenly stopping megadoses could potentially lead to 'rebound scurvy.' However, this has not been scientifically proven and is not a common concern for most individuals.