Protocols › Vitamin C

Vitamin C for colds

Goal: To make colds shorter and milder, as trials of daily vitamin C found.

Each protocol describes what studies used, for one goal or for one group of people. It is not advice for you: talk to a doctor before you start a supplement.

Updated 3 Nov 2026
Dose

At least 200 mg a day in the Cochrane review; 1 g a day or more in the 2023 review of severity.[1][2]

When to take

Every day, before a cold begins. Started only after symptoms began, it had no consistent effect.[1]

What studies found

A 2013 Cochrane review: colds were 8% shorter in adults and 14% shorter in children (31 comparisons, 9,745 colds). It did not prevent colds in the general population (10,708 people), but halved them in 598 marathon runners, skiers and soldiers.[1] A 2023 meta-analysis of 10 trials: colds 15% less severe.[2]

Ask a doctor first

High doses can cause diarrhea, nausea and stomach cramps. The US upper limit for adults is 2,000 mg a day, and lower for children.[3] In men, supplements of 1,000 mg a day or more went with more kidney stones.[4] Ask a doctor or pharmacist first if you have kidney stones, kidney disease or hemochromatosis, or are in cancer treatment.[3]

References

  1. Vitamin C for preventing and treating the common cold. Hemilä H, Chalker E Cochrane Database Syst Rev, 2013. PMID 23440782
  2. Vitamin C reduces the severity of common colds: a meta-analysis. Hemilä H, Chalker E BMC Public Health, 2023. PMID 38082300
  3. Vitamin C: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements. ods.od.nih.gov
  4. Total, Dietary, and Supplemental Vitamin C Intake and Risk of Incident Kidney Stones. Ferraro PM, Curhan GC, Gambaro G, Taylor EN Am J Kidney Dis, 2016. PMID 26463139