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# Methylcobalamin
## Overview
Methylcobalamin is the neurologically active form of Vitamin B12 (cobalamin). Unlike cyanocobalamin, it does not require conversion by the liver to become active and can act directly in the central nervous system. It serves as a cofactor for methionine synthase, essential for DNA and protein synthesis and nerve myelin repair.
## Primary Indications
* Peripheral neuropathy (diabetic or toxic)
* Vitamin B12 deficiency (pernicious anemia and dietary deficiency)
* Adjunctive therapy in megaloblastic anemia
* Hyperhomocysteinemia
## Adult Dosing
* **Peripheral Neuropathy:** 500 mcg orally three times daily (1,500 mcg total/day).
* **B12 Deficiency/Megaloblastic Anemia:** Dosing varies by severity. Oral replacement typically ranges from 1,000 mcg to 5,000 mcg daily depending on malabsorption severity.
* **Parenteral (IM/IV):** 500 mcg to 1,000 mcg administered 1–3 times weekly for severe deficiency or malabsorption.
## Pediatric Dosing
* **General Deficiency:** 100 mcg to 500 mcg orally daily, depending on age and severity.
* **Note:** Pediatric dosing is highly dependent on institutional protocols and underlying metabolic conditions (e.g., cobalamin metabolism disorders). Consult a specialist for specific pediatric metabolic dosing.
## Dose Adjustments
* **Renal/Hepatic Impairment:** No standard dose reduction required. However, maintenance in end-stage renal disease may require adjusted frequency based on serum B12/methylmalonic acid (MMA) levels.
## Contraindications
* Hypersensitivity to cobalt or cobalamin.
* Leber's hereditary optic neuropathy (administration may cause rapid optic nerve atrophy).
## Adverse Effects
* **Common:** Rash, nausea, diarrhea, headache.
* **Serious:** Anaphylaxis (rare), hypokalemia (during initial treatment of severe megaloblastic anemia due to sudden increase in erythropoiesis).
## Key Drug Interactions
* **Chloramphenicol:** May decrease the hematopoietic response to methylcobalamin.
* **Metformin/PPIs/H2 Antagonists:** Chronic use may reduce oral absorption of B12; monitoring of serum levels is recommended.
* **Colchicine/Neomycin:** May interfere with the gastrointestinal absorption of B12.
## Monitoring
* **Baseline:** Complete Blood Count (CBC), Reticulocyte count, serum B12, and Homocysteine levels.
* **During Treatment:** Monitor for resolution of neurological symptoms and normalization of Mean Corpuscular Volume (MCV). In severe anemia, monitor potassium levels during the first 48 hours of treatment.
## Clinical Pearls
* **Bioavailability:** Oral bioavailability of methylcobalamin is low due to the intrinsic factor mechanism; high-dose oral therapy (1,000+ mcg) is often used to ensure absorption via passive diffusion in malabsorptive states.
* **Neuropathy vs. Anemia:** Neurological symptoms (paresthesia, gait instability) caused by B12 deficiency may be irreversible if not treated promptly, regardless of hematological improvement.
* **Storage:** Methylcobalamin is light-sensitive; oral and injectable formulations must be protected from light to prevent degradation.
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**Educational Disclaimer:** This information is for educational purposes only and does not replace professional clinical judgment. Always verify dosages, contraindications, and drug interactions against current institutional protocols, local prescribing guidelines, and the manufacturer’s package insert before prescribing or administering medication.
