Vitamin C can help prevent infections and shorten their duration, but only if taken in the correct dose.

A large amount of research has been conducted on the ability of vitamin C to affect the common cold. Most of the studies used a daily dose of one gram, which is quite a small dose of vitamin C. Dr. Harri Hemilä from the University of Helsinki, Finland, analyzed the findings of a number of trials which investigated the effects of vitamin C doses on the duration of the common cold. The studies showed a significant dose-dependent relationship between the vitamin C and illness duration. Best results were seen in trials that used 6-8 grams per day.

Dr. Hemilä concluded the study by stating, "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 8 g/day vitamin C is beneficial for them. Self-dosing of vitamin C must be started as soon as possible after the onset of common cold symptoms to be most effective.”

It is not possible to get 8 grams of vitamin C from food when you consider that an orange only contains up to 70 milligrams, if it’s a very fresh orange. The longer fruits and vegetables are in storage, the more vitamin C is lost. Vitamin C is a very delicate vitamin and it is also destroyed by heat. The best way to take a vitamin C supplement is in divided doses. Taking 8 grams at once can cause diarrhea. It is best to split the 8 grams into 2 gram doses, taken 4 times per day.