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# Methylcobalamin
## Overview
Methylcobalamin is a biologically active, coenzyme form of Vitamin B12. Unlike cyanocobalamin, it does not require metabolic conversion in the liver to become active. It serves as a cofactor for methionine synthase, which converts homocysteine to methionine, and is essential for DNA synthesis and nerve tissue integrity.
## Primary Indications
* Vitamin B12 deficiency (peripheral neuropathy, megaloblastic anemia).
* Adjunctive treatment for diabetic neuropathy.
* Supportive care for subacute combined degeneration of the spinal cord.
## Adult Dosing
* **Peripheral Neuropathy:** Typically 1,500 mcg daily, administered in three divided doses (500 mcg three times daily).
* **Maintenance/Deficiency:** Dosing varies widely by local protocol and severity; oral supplementation often ranges from 500 mcg to 1,000 mcg daily.
* **Parenteral:** Severe deficiency may require 500 mcg IM/IV 1–3 times weekly depending on serum B12 levels and clinical response.
## Pediatric Dosing
* **General Deficiency:** No standard universal pediatric dosing exists. Consult local pediatric protocols or institutional guidelines.
* **Metabolic Disorders (e.g., Methylmalonic Acidemia):** Dosing is highly individualized and significantly higher (mg/day range) based on specialists' guidance.
## Dose Adjustments
* **Renal/Hepatic Impairment:** No specific standard dose reduction required as B12 is water-soluble and renally excreted in excess; however, monitor closely in patients with severe renal failure.
## Contraindications
* Hypersensitivity to cobalt or any component of the formulation.
* **Note:** Not appropriate as a monotherapy for Leber's optic atrophy (can cause severe/rapid vision loss).
## Adverse Effects
* **Common:** Injection site reactions (if IM), mild diarrhea, itching, or skin rash.
* **Rare/Serious:** Anaphylaxis (hypersensitivity), hypokalemia (during initial treatment of megaloblastic anemia due to rapid erythropoiesis).
## Key Drug Interactions
* **Chloramphenicol:** May diminish the hematopoietic effect of vitamin B12.
* **Metformin/Proton Pump Inhibitors/H2 Blockers:** May decrease oral absorption of B12; consider parenteral supplementation if deficiency is documented.
* **Colchicine/Neomycin:** May reduce oral absorption of B12.
## Monitoring
* **Baseline:** Serum B12 levels, complete blood count (CBC) to assess MCV/hemoglobin, and folate levels.
* **Follow-up:** Serum potassium (especially if treating initial deficiency), repeat CBC and serum B12 concentrations to confirm treatment response.
## Clinical Pearls
* **Superiority:** Some literature suggests methylcobalamin is more effective at targeting nerve tissue compared to cyanocobalamin, though clinical evidence for neuropathy remains mixed.
* **Administration:** Oral administration is often sufficient for dietary deficiency, but malabsorptive states (e.g., pernicious anemia, gastric bypass) typically require IM/IV or high-dose oral protocols.
* **Stability:** Protect from light; methylcobalamin is light-sensitive and degrades upon exposure.
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**Disclaimer:** This information is for educational purposes for healthcare professionals. Clinical practice varies by institution; always verify specific dosing, safety guidelines, and metabolic requirements against the latest prescribing information, hospital formulary protocol, or clinical lead before administration.
