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# Methylcobalamin
## Overview
Methylcobalamin is a biologically active form of Vitamin B12 (cobalamin). It acts as a coenzyme in the conversion of homocysteine to methionine and is essential for DNA synthesis, neurological function, and myelin formation. Unlike cyanocobalamin, it does not require initial hepatic conversion.
## Primary Indications
* Peripheral neuropathy (diabetic or toxic).
* Vitamin B12 deficiency (megaloblastic anemia).
* Neurological disorders associated with B12 deficiency.
## Adult Dosing
* **Peripheral Neuropathy:** 500 mcg orally three times daily.
* **B12 Deficiency (Oral):** 500–1,000 mcg daily, titrated based on serum levels.
* **Severe Deficiency (IM Protocol):** Dependent on local protocol; often 500–1,000 mcg intramuscularly once or twice weekly initially, tapering based on clinical response.
## Pediatric Dosing
* **General Deficiency:** No standard universal pediatric guideline exists for methylcobalamin. Dosing is highly variable and clinical-protocol dependent. Generally, oral supplementation ranges from 100–500 mcg daily depending on age and severity of deficiency, under specialist guidance.
## Dose Adjustments
* **Renal/Hepatic Impairment:** No formal dosage adjustments established; use with caution. Excess methylcobalamin is primarily excreted renally.
## Contraindications
* Hypersensitivity to cobalt or cobalamin products.
* Leber's hereditary optic neuropathy (may lead to rapid optic atrophy).
## Adverse Effects
* **Common:** Injection site pain (IM formulation), diarrhea, nausea, headache.
* **Rare:** Anaphylaxis, hypersensitivity reactions, rash.
## Key Drug Interactions
* **Chloramphenicol:** May decrease the hematopoietic response to B12.
* **Proton Pump Inhibitors (PPIs) / H2 Antagonists:** May reduce B12 absorption if provided orally; parenteral administration is preferred in patients on long-term acid suppression.
* **Metformin:** May inhibit gastrointestinal absorption of B12.
## Monitoring
* **Serum B12 levels:** Obtain baseline and follow-up.
* **Complete Blood Count (CBC):** Monitor MCV and hemoglobin levels in patients with megaloblastic anemia.
* **Homocysteine/Methylmalonic Acid (MMA):** Sensitive markers for evaluating functional B12 status if deficiency is suspected but serum B12 is borderline.
## Clinical Pearls
* **Bioavailability:** Methylcobalamin is retained better in tissues than cyanocobalamin but is more sensitive to light; store in light-protected containers.
* **Route:** Oral methylcobalamin is effective for correcting deficiency in patients with intact intrinsic factor function; patients with malabsorption (gastrectomy, Crohn's, pernicious anemia) require IM administration or high-dose sublingual/oral replacement.
* **Neuro-regeneration:** While frequently prescribed for neuropathy in clinical practice, evidence for symptomatic relief in non-B12-deficient diabetic neuropathy is considered weak and inconsistent.
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**Educational Disclaimer:** This information is for educational purposes only. Drug dosing, indications, and safety profiles can change. Always verify current prescribing information in a trusted clinical resource (e.g., Lexicomp, UpToDate) and consult local institutional protocols before prescribing or administering medication.
