Vitamin B12

There is strong, consistent evidence from authoritative bodies (NIH Office of Dietary Supplements, Cochrane) and systematic reviews that vitamin B12 (cobalamin) deficiency is well-characterized in specific risk groups and effectively treatable. Deficiency disproportionately affects vegans/vegetarians (natural B12 is limited to animal foods), older adults (3-43% of community-dwelling seniors, largely via atrophic gastritis/food-bound malabsorption), long-term users of metformin (dose-dependent reduction) and proton-pump inhibitors/H2 blockers, and people with pernicious anemia (autoimmune loss of intrinsic factor, the most common cause of clinically evident deficiency worldwide). Untreated deficiency can cause neurological damage (peripheral neuropathy, cognitive impairment, subacute combined degeneration) that may become irreversible, and neurological symptoms can precede or occur without anemia. On treatment, evidence supports that high-dose oral B12 works even when intrinsic-factor-mediated absorption fails, because roughly 1% of an oral dose is absorbed passively by mass action independent of intrinsic factor; thus ~1000-2000 mcg/day oral delivers therapeutic amounts. A 2018 Cochrane review found high-dose oral B12 similar to intramuscular injection for normalizing serum B12 (low-quality evidence, lower cost), and a 2025 systematic review/meta-analysis found sublingual and oral routes as effective as IM. On chemical form, NIH ODS states no evidence indicates absorption rates differ by supplement form, and cyanocobalamin is readily converted in the body to the active coenzyme forms, supporting broad equivalence of cyanocobalamin and methylcobalamin for correcting deficiency.