Vitamin C (ascorbic acid) is a water soluble antioxidant vitamin that is present in some foods, and which is a popular dietary supplement. Clinical studies in the effectiveness of supplementary Vitamin C to prevent and treat allergies in humans are mixed.
A 2011 meta-analysis of studies did not find any evidence supporting the use of Vitamin C to prevent or manage allergies.
However, a 2014 study which looked at blood serum levels of Vitamin C in asthmatic children found that "a relatively low dietary intake of vitamin C is associated with an increased risk of asthma and wheezing. Moreover, asthmatic patients who consumed vitamin C exhibited an improvement in their diseases."
Some studies have found that oxygen free radicals have a causal role in auto-immune diseases, such as rheumatoid arthritis (RA).
However, a report by the Harvard School of Public Health, found that "Randomized, placebo-controlled trials—which, when performed well, provide the strongest evidence—offer little support that taking vitamin C, vitamin E, beta-carotene, or other single antioxidants provides substantial protection against heart disease, cancer, or other chronic conditions. The results of the largest such trials have been mostly negative."
Vitamin C supplements come in immediate and time-released tablets, powders and creams.