Vitamin C

The short answer

Taking vitamin C every day does not reduce how often you catch a cold in the general population. It does modestly and consistently shorten a cold once you have one. Starting a large dose after symptoms begin has not shown consistent benefit.

The form, and why it matters

Ascorbic acid is the plain, cheapest and best-studied form, and it is what nearly all of the trial evidence used. Mineral ascorbates are buffered and gentler on an empty stomach, which matters to some people and changes nothing about the dose you need.

Typical dose

The adult reference intake sits near 75 to 90 mg a day, with a tolerable upper intake of 2,000 mg. The Cochrane review that underpins most of the cold evidence looked at trials using at least 200 mg a day. Moat carries 180 mg, which is the everyday nutritional amount rather than a megadose.

What the research shows

A Cochrane review pooled decades of placebo-controlled trials, 29 comparisons and more than 11,000 people, and found no reduction in cold incidence with regular supplementation. Duration came down modestly and consistently. Therapeutic doses started after symptom onset showed no consistent benefit.

Who should be careful

Above about 2,000 mg a day, stomach upset and diarrhoea become common. High-dose supplements can worsen iron overload in people with haemochromatosis, so check with a clinician if that applies to you.

Where it sits in our formulas

Sources

• Hemilä and Chalker, Vitamin C for preventing and treating the common cold, Cochrane Database of Systematic Reviews
• NIH Office of Dietary Supplements, Vitamin C fact sheet

Figures checked against the sources above on 21 September 2026.