Overview
Vitamin B12 (Cyanocobalamin/Mecobalamin) is an essential water-soluble vitamin that plays crucial roles in red blood cell formation, neurological function, and DNA synthesis. Cyanocobalamin is a synthetic form that converts to active forms in the body, while mecobalamin is an active form (methylcobalamin) available as a pharmaceutical preparation. Deficiency can lead to megaloblastic anemia and neurological disorders.
Mechanism of Action
Vitamin B12 acts as a cofactor for two essential enzymes: methionine synthase (involved in DNA synthesis and homocysteine metabolism) and L-methylmalonyl-CoA mutase (involved in fatty acid and amino acid metabolism). Cyanocobalamin is converted in the body to methylcobalamin and adenosylcobalamin, the two active coenzyme forms. Mecobalamin is already in the active methylcobalamin form and participates directly in neuronal myelination and neurotransmitter synthesis.
Indications
- Vitamin B12 deficiency (pernicious anemia, malabsorption syndromes)
- Megaloblastic anemia due to B12 deficiency
- Neuropathy (diabetic neuropathy, peripheral neuropathy)
- Nutritional supplementation
- Prophylaxis in vegetarians/vegans
- Certain neurological disorders
Dosage
Dosage varies based on formulation and indication. Typical adult doses: Cyanocobalamin injection: 100-1000 mcg IM daily for 5-10 days, then monthly. Oral: 1000-2000 mcg daily for deficiency. Mecobalamin: 500 mcg three times daily for neuropathy. Always follow prescribing information and adjust based on clinical response.
Contraindications
- Hypersensitivity to cyanocobalamin, mecobalamin, or any component
- Leber's disease (hereditary optic nerve atrophy)
- Cobalt allergy
Side Effects
- Injection site reactions (pain, swelling)
- Mild diarrhea
- Itching
- Rash
- Headache
- Nausea
- Hypokalemia
- Peripheral vascular thrombosis
- Anaphylaxis (rare)
- Pulmonary edema (with rapid IV administration)
Interactions
- Chloramphenicol: May decrease B12 efficacy
- Proton pump inhibitors/H2 blockers: May reduce absorption
- Metformin: Long-term use may decrease B12 absorption
- Aminoglycosides, colchicine: May reduce absorption
- Alcohol: Chronic use impairs absorption
- Oral contraceptives: May decrease serum B12 levels