A method and medical composition for the treatment and/or prevention of a functional Vitamin B12 deficiency in an individual that is brought about as a consequence of oxidative stress on biochemical pathways. The functional Vitamin B12 deficiency may eventually present as dementia, other neuropsychiatric abnormality and/or vascular disease. The method involves the administration of a medical composition that supplies a cobalt-sulphur bond in the upper β-ligand of an intracellular cobalamin molecule thereby facilitating intracellular processing of cobalamin. The cobalt-sulphur bond may be provided directly by administration of a thiolatocobalamin, such as glutathionyl-cobalamin or indirectly by the co-administration of Vitamin B12 (or a derivative thereof) with a sulphur-containing molecule, such as glutathione or a precursor thereof.