Is there a downside to taking vitamin C?

December 30, 2025 •

3 min read

While vitamin C is a crucial nutrient, most adults have a tolerable upper intake level of 2,000 mg per day. Consuming high doses, often from supplements, can lead to side effects and is why many people question if there is a downside to taking vitamin C.

Quick Summary

Taking excessive vitamin C, primarily via supplements, can cause gastrointestinal distress and increase the risk of kidney stones. High intake is particularly concerning for those with specific health conditions, such as iron overload.

Key Points

Digestive Distress: High doses of vitamin C from supplements can cause nausea, diarrhea, stomach cramps, and heartburn.
Kidney Stone Risk: Megadosing on vitamin C increases the risk of calcium oxalate kidney stones, especially in men and individuals with a history of them.
Iron Overload Concern: Vitamin C enhances iron absorption, posing a risk to those with iron overload disorders like hemochromatosis.
Medication Interference: Excessive vitamin C can interact with and affect the efficacy of certain medications, including blood thinners and chemotherapy.
Upper Intake Limit: For most adults, the safe daily upper limit for vitamin C intake is 2,000 mg; intake beyond this increases the likelihood of side effects.
Dietary Sources are Safer: It is nearly impossible to get too much vitamin C from food alone, making dietary intake the safest option.

The Potential Drawbacks of High-Dose Vitamin C Intake

Vitamin C, or ascorbic acid, is a water-soluble vitamin vital for immune function, collagen synthesis, and iron absorption. While obtaining enough from a balanced diet is ideal, many take high-dose supplements. Unlike fat-soluble vitamins, excess water-soluble vitamin C is generally excreted. However, this does not mean high intake is without risks. Consuming large quantities, particularly from supplements, can result in various side effects.

Common Gastrointestinal Side Effects

Excessive vitamin C commonly causes digestive issues due to the body's inability to absorb large amounts, leading to fermentation in the gut. Symptoms may include:

Nausea and vomiting
Diarrhea
Stomach cramps and bloating
Heartburn, especially on an empty stomach

These side effects are more likely and severe above the adult Tolerable Upper Intake Level (UL) of 2,000 mg per day.

Increased Risk of Kidney Stones

A significant risk of high-dose vitamin C supplementation is an increased chance of kidney stones. Excess vitamin C is converted to oxalate, a waste product. High oxalate levels in the kidneys can combine with calcium to form painful crystals. Studies suggest a link between high-dose supplements (over 1,000 mg/day) and increased risk of calcium oxalate kidney stones, especially for men and those with a history of stones.

Specific Health Concerns and Drug Interactions

While most healthy individuals can handle excess vitamin C, certain conditions make high intake risky:

Iron Overload: In hemochromatosis, where the body stores excess iron, vitamin C's iron-enhancing effect is dangerous, potentially causing organ damage.
G6PD Deficiency: High doses can cause red blood cell breakdown in individuals with this disorder.
Chronic Kidney Disease: Those with impaired kidney function may struggle to filter excess oxalate, potentially worsening kidney issues.
Medication Interactions: High vitamin C can interfere with medications like blood thinners and chemotherapy. Consultation with a doctor is advised if taking medication and considering high-dose supplements.

Rebound Scurvy

A rare occurrence, rebound scurvy, can affect infants whose mothers took very high vitamin C doses during pregnancy, leading to deficiency symptoms after birth due to the sudden drop in intake.

Excessive Intake vs. Recommended Intake

Understanding the difference between excessive and recommended amounts is crucial:


Aspect	Excessive Intake (>2,000 mg/day)	Recommended Daily Allowance (RDA)	Tolerable Upper Intake Level (UL)
Source	Primarily high-dose supplements	A variety of fruits and vegetables	The maximum intake considered safe
Potential Risks	Gastrointestinal issues, kidney stones, iron overload, drug interactions	Minimal to no risks in healthy individuals	Risk of side effects increases above this level
Absorption	Body struggles to absorb large quantities	Absorbed efficiently from food	Limited absorption beyond a certain point
Excretion	Processed and excreted, with excess converted to oxalate	Easily excreted in urine	Excreted, but with greater risk of oxalate buildup

Conclusion: Moderation is Key

The question, is there a downside to taking vitamin C, is valid primarily when considering excessive supplement use. The risks of digestive upset, kidney stones, and other issues are linked to megadosing, not a diet rich in vitamin C foods. To avoid negative consequences while enjoying vitamin C's benefits, prioritize dietary sources and avoid high-dose supplements unless advised by a healthcare provider. Consult a doctor about your intake if you have concerns or pre-existing conditions. Further information on nutrient guidelines is available on the NIH Office of Dietary Supplements website.

How to Manage Potential Side Effects

If you experience side effects from a vitamin C supplement, consider these steps:

Reduce the dose: Lowering the amount often helps alleviate symptoms like stomach upset.
Take with food: Consuming the supplement with a meal can lessen gastric irritation and heartburn.
Choose buffered supplements: For stomach sensitivity, buffered forms like calcium or sodium ascorbate are less acidic and may be easier to tolerate.
Stay hydrated: Drinking sufficient water can aid in flushing out excess oxalate, potentially reducing kidney stone risk.
Prioritize diet: Obtaining vitamin C from foods like citrus fruits, peppers, broccoli, and strawberries is the safest way to meet your needs.

These measures can help ensure safe and effective vitamin C intake.

Frequently Asked Questions

Serious health problems from too much vitamin C are rare in healthy individuals because the body excretes the excess. However, high doses can lead to uncomfortable digestive issues, and in predisposed individuals, may increase the risk of kidney stones or exacerbate conditions like hemochromatosis.

The tolerable upper intake level (UL) for adults is 2,000 milligrams (mg) per day. Taking more than this increases the risk of side effects.

Yes, digestive issues like diarrhea, nausea, and stomach cramps are the most common side effects of taking high doses of vitamin C supplements. This is because the body cannot absorb all of the excess at once.

No, it is nearly impossible to consume too much vitamin C from food alone. The negative side effects are almost exclusively associated with taking high-dose supplements.

Individuals with a history of kidney stones, chronic kidney disease, hemochromatosis (iron overload), or G6PD deficiency should be cautious with high vitamin C intake. People on certain medications should also consult a doctor.

When taken in high doses, excess vitamin C is metabolized into oxalate. In some people, this oxalate can bind with calcium in the kidneys and form painful calcium oxalate crystals, or kidney stones.

Yes, buffered forms of vitamin C like calcium ascorbate are less acidic and may be less likely to cause heartburn or stomach irritation compared to standard ascorbic acid supplements, which can be harsh on a sensitive stomach.