Vitamin B12 / cyanocobalamin / methylcobalamin

Vitamin B12 supports blood cell production and nerve function. Deficiency develops slowly and can cause anaemia, tingling, balance problems and cognitive changes, some of which become irreversible if untreated. Supplement labels use several forms — cyanocobalamin, methylcobalamin, hydroxocobalamin and adenosylcobalamin.

B12 is a large cobalt-containing molecule. Cyanocobalamin is stable and inexpensive and is the form used in most research and fortification. Methylcobalamin and adenosylcobalamin are coenzyme forms, and hydroxocobalamin is often used in injections.

B12 acts as a cofactor for two enzymes: one converts homocysteine to methionine, the other supports energy metabolism from fats and some amino acids. Both matter for red blood cell production and myelin maintenance.

Treating deficiency reliably corrects anaemia and can improve or halt neurological symptoms, with best results when treatment starts early. Trials of B vitamin supplementation to prevent cognitive decline or cardiovascular events in people who are not deficient have generally been negative or inconsistent, even though homocysteine falls.

Laboratory work describes cobalamin transport proteins, cellular uptake and the biochemical consequences of depletion in cultured cells and animal models.

Evidence gap: Non-human research. It explains mechanism and does not establish additional human benefits beyond correcting deficiency.

Absorption depends on stomach acid and intrinsic factor, so acid-reducing medicines, metformin, gastric surgery and age-related changes reduce it. Excess is largely excreted in urine.

B12 has low toxicity and no established upper intake level, but that does not make it universally appropriate. Deficiency must be diagnosed rather than assumed, because the cause changes the treatment; neurological symptoms need prompt evaluation. Rare allergic reactions to injections occur. Starting folate or B12 without testing can obscure the diagnosis.

Tablets, sublingual tablets, liquids and injections are all used. Sublingual and oral forms perform similarly in most studies.

Supplementation is primarily for preventing or correcting deficiency. Related approaches include dietary or fortified sources, reviewing medicines that reduce absorption, and investigating causes such as pernicious anaemia or coeliac disease. It is not a treatment for fatigue of unknown cause.

Educational example: an adult on long-term acid suppression takes a high-dose B-complex, feels no different, and never has a level or a cause checked. Decision factors are whether deficiency is documented, what is causing it, and whether neurological symptoms are present. Useful questions for a clinician or pharmacist: should I be tested, is my medicine contributing, and do I need oral or injected treatment? Labels emphasising 'energy' go beyond what evidence supports in people with normal status.