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Vitamin C

Also known as: Ascorbic acid

An essential antioxidant vitamin. Regular intake has a small effect on cold duration; it doesn't prevent colds for most people.

Common forms: Tablets · Powder · Chewables

Draft summary, not yet reviewed by a qualified health professional · Last updated 2026-11-03

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This information is for education only and is not medical advice, diagnosis, or treatment. Supplements can interact with medications and health conditions. Check with your doctor or pharmacist before starting anything, especially if you take medication, are pregnant or breastfeeding, or have a health condition. These statements have not been evaluated by the FDA; supplements are not intended to diagnose, treat, cure, or prevent any disease.

What the evidence says

Colds & immune support

CLimited evidence

A Cochrane review found regular vitamin C did not reduce how often most people catch colds but slightly shortened their duration (about 8% in adults). It roughly halved cold risk in people under heavy physical stress (e.g. marathon runners).

Dose used in studies
200–1,000 mg/day taken regularly
Time to effect
Needs regular use before colds start
Most relevant for
People under heavy physical stress
Studies (1)

Safety

Common side effects

  • Diarrhea or stomach upset at high doses

Avoid or ask your doctor first if

  • You have a history of kidney stones (high doses)
  • You have hemochromatosis

Pregnancy & breastfeeding

Likely safe at normal dietary doses — confirm with your provider.

Upper limit

2,000 mg/day

Medication interactions

No well-documented interactions with common medication classes. Still tell your pharmacist what you take.

This list covers common medication classes only and is not exhaustive.

Buying tips

Look for third-party testing seals (USP Verified, NSF Certified, or Informed Choice) and match the form and dose used in studies.

Where to buy (affiliate)

Further reading