Vitamin C (ascorbic acid) is a water-soluble vitamin, which is needed by the body to form collagen in bones, cartilage, muscle, and blood vessels, and which aids in the absorption of iron. Dietary sources of vitamin C include fruits and vegetables, particularly citrus fruits such as oranges.
 
Severe deficiency of vitamin C causes scurvy. Although rare, scurvy includes potentially severe consequences and can cause sudden death. Patients with scurvy are treated with vitamin C and should be under medical supervision.
 
Many uses for vitamin C have been proposed, but few have been found to be beneficial in scientific studies. In particular, research on asthma, cancer, and diabetes remains inconclusive, and no benefits have been found for the prevention of cataracts or heart disease.
 
The use of vitamin C in the prevention or treatment of the common cold and respiratory infections remains controversial, with research ongoing. For cold prevention, more than 30 clinical trials including over 10,000 participants have examined the effects of taking daily vitamin C. Overall, no significant reduction in the risk of developing colds has been observed. In people who developed colds while taking vitamin C, no difference in the severity of symptoms has been seen overall, although a small, significant reduction in the duration of colds has been reported (approximately 10% in adults and 15% in children). Notably, some studies of people living in extreme circumstances, including soldiers in subarctic exercises, skiers, and marathon runners, have found a significant reduction in the risk of developing a cold, by approximately 50%. This area merits additional research and may be of particular interest to elite athletes or military personnel. For cold treatment, numerous studies have examined the effects of starting vitamin C after the onset of cold symptoms. So far, significant benefits have not been observed.