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# Methylcobalamin
## Overview
Methylcobalamin is a neurologically active form of Vitamin B12 (cobalamin). It acts as a cofactor for methionine synthase, facilitating the conversion of homocysteine to methionine and supporting myelin sheath maintenance.
## Primary Indications
* Peripheral neuropathy.
* Vitamin B12 deficiency (megaloblastic anemia).
* Adjunctive therapy for diabetic neuropathy.
## Adult Dosing
* **Peripheral Neuropathy:** 1500 mcg daily, administered in three divided doses (500 mcg TID).
* **Maintenance/Prophylaxis:** Dosing varies significantly based on physician preference and baseline serum levels; typical ranges are 500–1000 mcg daily or weekly depending on clinical severity.
* *Note: Oral bioavailability is low; parenteral administration is preferred for malabsorption syndromes.*
## Pediatric Dosing
* **Deficiency:** No standardized pediatric dosing exists. Doses vary by institutional protocol. Common ranges include 100–500 mcg orally daily or periodic intramuscular injections based on hematologic response. Consult local pediatric hematology guidelines.
## Dose Adjustments
* **Renal Impairment:** No formal dosage adjustments required, though patients with chronic kidney disease may require higher doses to overcome secondary deficiencies.
* **Hepatic Impairment:** No formal dosage adjustments required.
## Contraindications
* Hypersensitivity to cobalt or any component of the formulation.
* *Leber’s Hereditary Optic Atrophy:* Use with extreme caution as rapid administration of B12 may cause severe and swift optic nerve atrophy.
## Adverse Effects
* **Common:** Injection site pain (parenteral), rash, pruritus.
* **Rare/Serious:** Anaphylaxis, hypokalemia (during early treatment of megaloblastic anemia due to intense erythropoiesis), and polycythemia vera symptom exacerbation.
## Key Drug Interactions
* **Chloramphenicol:** May decrease the hematologic response to Vitamin B12.
* **Proton Pump Inhibitors/H2 Antagonists/Metformin:** May decrease oral absorption of Vitamin B12.
* **Alcohol:** Chronic heavy intake may interfere with B12 absorption.
## Monitoring
* **Baseline:** Serum B12 levels, Complete Blood Count (CBC) with peripheral smear, and serum potassium (if treating significant deficiency).
* **Follow-up:** Reticulocyte count (roughly 5–7 days after starting treatment for anemia) and symptomatic relief of neuropathy.
## Clinical Pearls
* Methylcobalamin is often marketed as "bio-identical" or superior to cyanocobalamin, but clinical outcomes for systemic deficiency are largely similar across forms; cyanocobalamin remains the standard for most parenteral protocols.
* Always evaluate for underlying causes of deficiency (e.g., pernicious anemia, dietary restriction, malabsorption) rather than treating with supplements alone.
* Serum B12 levels may not always reflect intracellular status.
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**Educational Disclaimer:** This information is for educational purposes only. Clinical practice varies by region and institutional protocol. Always verify exact dosages, contraindications, and drug interactions using current, peer-reviewed clinical databases (e.g., Lexicomp, UpToDate) or the official product labeling before prescribing or administering medication.
