Hematinic drugs: Vitamin B12

Vitamin B₁₂

Vitamin B₁₂ preparations are used to treat pernicious anemiaCommon vitamin B₁₂ preparations include cyanocobalamin andhydroxocobalamin.

Pharmacokinetics

Vitamin B₁₂ is available in parenteral, oral, and intranasal forms. For the body to absorb oral forms of vitamin B₁₂, the gastric mu-cosa must secrete a substance called intrinsic factor. People who have a deficiency of intrinsic factor develop a special type of ane-mia known as vitamin B₁₂-deficiency pernicious anemia.

Parenteral possibilities

When cyanocobalamin is injected by the I.M. or subcutaneous (subQ) route, it’s absorbed and bound to transcobalamin II for transport to the tissues. It then travels via the bloodstream to the liver, where 90% of the body’s supply of vitamin B₁₂ is stored.

Although hydroxocobalamin is absorbed more slowly from the injection site, its uptake in the liver may be greater than that of cyanocobalamin. Hydroxocobalamin is only administered I.M.

Most gets lost

With either drug, the liver slowly releases vitamin B₁₂ as needed by the body. About 3 to 8 mcg of vitamin B₁₂ are excreted in bile each day and then reabsorbed in the ileum. It’s also secreted in breast milk during lactation.

Within 48 hours after a vitamin B₁₂ injection, 50% to 95% of the dose is excreted unchanged in urine.

Pharmacodynamics

When vitamin B₁₂ is administered, it replaces vitamin B₁₂ that the body would normally absorb from the diet. This vitamin is essen-tial for cell growth and replication and for the maintenance of myelin (nerve coverings) throughout the nervous system. Vitamin B₁₂ may also be involved in lipid and carbohydrate metabolism.

Pharmacotherapeutics

Cyanocobalamin and hydroxocobalamin are used to treat perni-cious anemia, a megaloblastic anemia characterized by decreased gastric production of hydrochloric acid and intrinsic factor defi-ciency. Intrinsic factor, a substance normally secreted by the pari-etal cells of the gastric mucosa, is essential for vitamin B₁₂ absorp-tion. Intrinsic factor deficiencies are common in patients who have had total or partial gastrectomies or total ileal resection.Oral vitamin B₁₂ preparations are used to supplement nutri-tional deficiencies of the vitamin. The parenteral and intranasal formulations are used to treat patients with pernicious anemia.

Drug interactions

Alcohol, aspirin, neomycin, chloramphenicol, and colchicine may decrease the absorption of oral cyanocobalamin. (See Adverse re-actions to vitamin B₁₂ therapy.)