Class: Essential B Vitamin (B12)
Dosage Forms. Injection Solution: 1000 mcg/mL; Mucous Membrane Lozenge/Troche: 50 mcg, 100 mcg, 250 mcg, 500 mcg; Tablet: 50 mcg, 100 mcg, 250 mcg, 500 mcg, 1000 mcg; Tablet, Extended Release: 1000 mcg; Sublingual Tablet: 1000 mcg
Common FDA Label Indication, Dosing, and Titration.
1. Cobalamin deficiency, normal absorption: 1000 mcg po daily
2. Cobalamin deficiency, malabsorption: 100 mcg IM or deep subq injection daily for 6-7 d, then 100 mcg monthly for life
1. Hyperhomocysteinemia: 400 mcg po daily
MOA. B12 is required for the synthesis of the amino acid methionine from homocysteine. A deficiency of B12 results in hyperhomocysteinemia and a decrease in methionine. Since methionine is required for DNA synthesis, B12deficiency also results in decreased DNA synthesis, which presents clinically as macrocytic anemia when red blood cells are unable to extrude their nucleus.
Drug Characteristics: Cyanocobalamin
Medication Safety Issues: Cyanocobalamin
Drug Interactions: Cyanocobalamin. None known
Adverse Reactions: Cyanocobalamin
Efficacy Monitoring Parameters. Baseline and periodic B12 and folic acid levels, intrinsic factor, normalization of MCV, normalization of Hgb, resolution of symptoms of anemia (fatigue, shortness of breath).
Toxicity Monitoring Parameters. Seek medical attention if severe shortness of breath, swelling, skin rash, or hives.
Key Patient Counseling Points. May require several weeks for maximum effect. Take extended-release products with food. Avoid alcohol as it inhibits the absorption of B12.
Clinical Pearls. Drugs that interfere with folate metabolism (methotrexate, hydroxyurea, pemetrexed) will cause an elevated MCV in the absence of vitamin B deficiency. Patients on pemetrexed receive B12 to prevent toxicity. Metformin decreases B12.