Scientists have been debating the potential benefits of taking vitamin C for the common cold since the 1940s. Although research results are conflicting, one thing is clear -- taking vitamin C in any amount isn't going to immediately cure your cold. At best it may help reduce the severity of your symptoms, the number of colds you get or the duration of your cold.
Frequency of Colds
A meta-analysis published online in the The Cochrane Library in 2013 found that taking vitamin C regularly didn't decrease the likelihood of colds in the general population. Those who exercised in extreme conditions, such as skiers, soldiers in subarctic conditions and marathon runners, however, had a decrease in the number of colds they experienced of approximately 50 percent with the use of vitamin C.
Duration of Colds
A literature review published in the "Journal of the American Academy of Nurse Practitioners" in May 2009 concluded that taking vitamin C isn't likely to improve the symptoms of the common cold, but that it could shorten the duration of colds. Regularly taking vitamin C as a prophylaxis may shorten the duration of colds by about 8 percent in adults and about 14 percent in children, but taking vitamin C after the start of the cold doesn't appear to have this beneficial effect, notes the Office of Dietary Supplements.
Severity of Colds
A combination of 1,000 milligrams of vitamin C and 10 milligrams of zinc helped people with colds reduce their symptoms during five days of treatment compared to a placebo, according to a study published in the "Journal of International Medical Research" in February 2012. This benefit may be due to the zinc, however, as a study published in the "European Journal of Clinical Nutrition" in August 2005 found that taking 500 milligrams of vitamin C per day didn't lessen the severity of colds or their duration, although it did improve the likelihood of having fewer than three colds in five years.
Limiting Adverse Effects
Colds can make asthma worse by making the lungs temporarily more sensitive to histamine. Taking vitamin C may help reduce common-cold-induced asthma attacks, in part by reducing the lungs' sensitivity to histamine, according to a review article published in "Allergy, Asthma & Clinical Immunology" in 2013. The participants in the studies reviewed took 1 gram to 5 grams of vitamin C per day.