Overview
Vitamin B12, or cobalamin, is a water-soluble vitamin required for DNA synthesis, normal red-blood-cell formation and neurologic function. Deficiency can cause megaloblastic anemia, fatigue, neurologic symptoms and reduced functional capacity. Athletes following vegan or highly restrictive diets, older adults, people with pernicious anemia, gastrointestinal disease or surgery, and people using some medications may be at increased risk of low B12 status. In those settings, supplementation can be important. In contrast, available sports-nutrition evidence does not support using high-dose B12 as an energy or endurance booster when status is already adequate. B12 products contain different forms and doses, often far above the RDA because absorption becomes less efficient at high oral doses. The correct approach is to identify risk, confirm deficiency when appropriate and treat the underlying cause rather than using B12 as a generic fatigue supplement.
Common use
Preventing or correcting vitamin B12 deficiency or inadequate intake; not a general performance enhancer in B12-sufficient people.
Common forms
Cyanocobalamin and methylcobalamin tablets, capsules, lozenges and sublingual products; prescription oral or injectable forms are used in selected clinical situations.
Practical notes
Consider diet pattern, symptoms, medication use and absorption risk. Vegan diets require a reliable B12 source. Persistent fatigue, anemia or neurologic symptoms require appropriate medical assessment. Last evidence review: 14 August 2026.