Vit C and the Common Cold: What’s the Optimal Dose?

Study authors concluded that a "significant dose-response relationship" existed between the vitamin C dose and duration of the common cold.

A new analysis by the University of Helsinki suggests that the length of a common cold may be reduced by increasing the patient’s dose of daily vitamin C intake.

Most of the previous trials which have looked into human infections, such as the common cold, and vitamin C administration have focused on a modest dosage, usually just 1g/day. Pooled analyses of these studies in scientific literature have shown significant differences and improvements for vitamin C compared with placebo, pointing to a “genuine biological effect.” But the optimal doses and maximum effects of vitamin C on the common cold are unknown, explained Dr. Harri Hemilä from the University of Helsinki, Finland. 

To examine dose-responses, researchers assessed 2 randomized trials that investigated various vitamin C doses on the duration of the common cold. One trial (Anderson, 1974) randomized patients to vitamin C 4g/day, 8g/day and placebo on the first day of the patient’s cold. Results showed that compared with the placebo group, the 8g/day dose cut the duration of colds by 19%, twice as much as the 4g/day dose did (P=0.013).

The second trial (Karlowski, 1975), a 2×2 trial, randomized 3g/day of vitamin C in one group; 3g/day for 5 days in a second group when participants caught a cold; 6g/day to a third group; and placebo to a fourth group. Results demonstrated that compared to the placebo group, the 6g/day dose cut duration of colds by 17%, twice as much as the 3g/day doses did (P=0.018).  

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Study authors concluded both studies showed a “significant dose-response relationship” between the vitamin C dose and duration of the common cold, with a linear trend up to 6–8g/day. Higher doses of vitamin C may result in even shorter duration of the common cold, Dr. Hemilä added.

Other studies suggesting that the optimal dose of vitamin C should be >15g/day for the best treatment of common colds were also noted but the maximum doses evaluated in randomized trials have been much lower.

Dr. Hemilä concludes that, “given the consistent effect of vitamin C on the duration of colds, and its safety and low cost, it would be worthwhile for individual common cold patients to test whether therapeutic 8g/day vitamin C is beneficial for them.” Vitamin C should be started as soon as possible after the onset of cold symptoms to see the greatest benefit, the study concluded. 

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