Nitrous oxide may cause neurological and haematological signs and symptoms, as a result of its tendency to form complex with cobalt(I) in methylcobalamin, the cofactor for methionine synthase (EC 2.1.1.13), resulting in irreversible oxidation of the cofactor and inactivation of the enzyme protein. Formation of active enzyme requires new protein synthesis, as well as cobalamin supply (vitamin B12). Patients with low or marginal cobalamin depots (commonly found in the elderly) are at risk. The risk may be avoided by giving vitamin B12 to patients with confirmed or suspected cobalamin deficiency in good time before surgery involving nitrous oxide anaesthesia.