Vitamin B12 (cobalamin) is a water-soluble vitamin found naturally in animal foods. The body needs it for healthy blood and nerves. As a supplement it is used to prevent or treat low B12, for example in older adults, vegans, after weight-loss surgery, or with long-term metformin or acid-reducer use.
Also known as: cobalamin, cyanocobalamin, methylcobalamin.
ODS lists a Recommended Dietary Allowance of 2.4 mcg a day for adults, 2.6 mcg in pregnancy and 2.8 mcg while breastfeeding. There is no upper limit, because B12 is generally considered safe even at high doses. B12-only supplements typically contain 500 to 1,000 mcg.
It is sold in the US as a dietary supplement. Under the 1994 dietary supplement law (DSHEA), FDA does not approve dietary supplements before they are marketed, and the maker, not FDA, is responsible for safety and labeling. B12 is also a prescription drug: cyanocobalamin and hydroxocobalamin injections, usually into muscle, and a prescription nasal gel, used for deficiency such as from pernicious anemia.
ODS says the share of a dose absorbed falls as the dose rises: about 50% at 1 to 2 mcg, but only about 2% at 500 mcg and 1.3% at 1,000 mcg. A Cochrane review cited by ODS found very high oral doses (1,000 to 2,000 mcg) normalized blood B12 about as well as injections. ODS lists metformin and acid reducers (proton pump inhibitors such as omeprazole and H2 blockers) as medicines that can lower B12. Weight-loss surgery also reduces B12 absorption; ODS notes 1,000 mcg a day of oral methylcobalamin appeared as effective as injections after gastric bypass.
We could not find a single published half-life or time to peak for vitamin B12 that applies to a supplement dose, so we do not list one. B12 is stored in the body and absorbed through a carrier that saturates at small doses, so blood levels after one pill do not follow a simple curve. ODS gives absorption of about 2% at 500 mcg and 1.3% at 1,000 mcg.
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