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Vitamin B12 (cobalamin) helps your body make DNA, form healthy red blood cells, and keep nerves working. Low B12 can cause megaloblastic anemia and neurologic symptoms. People typically get B12 from animal foods or fortified foods; supplements are available if diet or absorption is an issue. For fundamentals, see the NIH Office of Dietary Supplements consumer fact sheet and the family‑medicine review on vitamin B12 deficiency.
For most adults, the Recommended Dietary Allowance (RDA) is 2.4 micrograms (mcg) per day (2.6 mcg if pregnant; 2.8 mcg if lactating). The RDA meets the needs of nearly all healthy people and assumes typical absorption from food. If you cannot absorb B12 well due to pernicious anemia or gastrointestinal surgery, you may need higher oral doses or prescription therapy (see cyanocobalamin injection).
Older adults often absorb less B12 from food because intrinsic‑factor–dependent absorption declines. Many will meet needs with fortified foods or a standard supplement that provides at least the RDA, but some need more to maintain normal levels. For who is at higher risk and when to test, see Yale Medicine’s overview.
High‑dose oral B12 is used when absorption is impaired because a small passive fraction is absorbed even without intrinsic factor. The NIH fact sheet for health professionals notes that absorption drops sharply once intrinsic‑factor capacity is exceeded (about 1–2% of large doses). Clinical Q&A from UCLA Health indicates that large supplemental doses are generally considered safe for most people, though persistent unexplained high blood levels warrant evaluation (see Testing & Normal Levels).
There is no established Tolerable Upper Intake Level (UL) for B12 due to low toxicity, but more is not automatically better. Studies show that markedly elevated plasma B12 can be a marker of underlying illness, and persistent high B12 has been linked with short‑term cancer detection and mortality in cohorts—likely reflecting reverse causation rather than harm from typical supplements. If your serum B12 is very high without supplementation, ask your clinician about evaluation.
When deficiency stems from pernicious anemia or significant malabsorption, injections (often cyanocobalamin 1,000 mcg IM per FDA labeling) are used to replete stores. A commonly used approach is a loading phase over several weeks followed by monthly maintenance when the cause persists. See MedlinePlus: cyanocobalamin injection for patient‑friendly details and discuss an individualized schedule with your clinician.
Cyanocobalamin, methylcobalamin, and hydroxocobalamin all work. For most people, form matters less than dose and adherence. Oral B12 from supplements is more bioavailable than food‑bound B12, and absorption decreases at higher single doses once intrinsic‑factor capacity is exceeded. For pragmatic primary‑care options, see BC provincial guideline on vitamin B12 and folate deficiency.
B12 is water‑soluble; excess is generally excreted. Typical side effects from supplements are uncommon and mild. Several medicines can lower B12 by impairing absorption. The FDA warns that metformin can cause low vitamin B12, and long‑term proton‑pump inhibitor use has been associated with increased risk of deficiency in observational studies. If you take these medicines and have compatible symptoms, ask about testing (see below).
B12 status is typically assessed with a serum B12 test. Many U.S. labs report a reference interval roughly around 232–1245 pg/mL (example: Labcorp). If your B12 is borderline or symptoms persist, clinicians often order methylmalonic acid (MMA), which rises when B12 is functionally low. See the AAFP clinical review for thresholds and when confirmatory tests are useful.
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NIH ODS. Vitamin B12 – Health Professional Fact Sheet. (2025).
FDA DailyMed. Cyanocobalamin Injection, Solution Label. (2022).
AAFP. Vitamin B12 Deficiency: Recognition and Management. (2017).
JNCI. Elevated Plasma Vitamin B12 as a Marker for Cancer. (2013).
Nature Scientific Reports. Persistent elevation of plasma vitamin B12… (2021).
BC Guidelines. Cobalamin (Vitamin B12) and Folate Deficiency. (2023).
FDA Drug Safety Communication. Metformin and Low Vitamin B12. (2020).
