Methylcobalamin is the active coenzyme form of vitamin B12, ready to participate in methylation reactions without requiring conversion.
Mechanism — Methyl donor in the methionine cycle, alongside methylfolate. Cofactor for methionine synthase and methylmalonyl-CoA mutase. Critical for myelin synthesis, DNA methylation, and red blood cell maturation.
Use case — Vegetarians/vegans (no plant source), elderly (absorption declines), MTHFR variants, anyone on metformin (depletes B12). Typical 1,000-5,000 mcg sublingual daily.
Caveats — Cyanocobalamin is the cheap form — fine for most but suboptimal for methylation issues. High doses are renally cleared; oversupplementation isn't dangerous but also isn't useful past a threshold. Check serum B12 and MMA for true status.