In humans, ascorbic acid, better known as vitamin C, is a true vitamin because humans lack the ability to synthesise it. Vitamin C exhibits a number of enzymatic and non-enzymatic effects but all are accounted for by the ability of vitamin C to donate electrons and therefore acts as a reducing agent. It has a wide range of functions. For example, it acts as co-factor for a number of enzymes including those involved in collagen hydroxylation, prevents oxidative damage to DNA and intracellular proteins, and in plasma it increases endothelium-dependent vasodilatation, and reduces extracellular oxidants from neutrophils. Deficiency in vitamin C results in the potentially fatal disease scurvy, which can be cured only by administering vitamin C. It has been shown that in individuals with gingivitis and periodontitis, plasma vitamin C levels are lower than in healthy controls. In periodontitis, a reduced capacity to absorb vitamin C may play a role. The manner in which vitamin C data are obtained from blood significantly impacts the final value obtained and therefore data validity. Plasma vitamin C levels of 56.8 μmol/l may be regarded as the optimum plasma level. In order to achieve this level, at least 200 mg vitamin C per day should be ingested. It is advisable to obtain vitamin C through the consumption of fruit and vegetables rather than supplements.