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# Methycobalamine (Methylcobalamin)
## Overview
- A coenzyme form of vitamin B12, essential for DNA synthesis, myelin formation, and neurological function.
- Available as oral tablets, sublingual, and injectable (IM/SC) formulations.
## Primary Indications
- Vitamin B12 deficiency (pernicious anemia, malabsorption, vegan diet)
- Peripheral neuropathy (diabetic, alcoholic, drug-induced)
- Adjunctive treatment for hyperhomocysteinemia
- Some protocols for cognitive decline or chronic fatigue (limited evidence)
## Adult Dosing
- **B12 deficiency with neurologic symptoms**: 1 mg IM/SC daily or every other day for 1–2 weeks, then 1 mg weekly until recovery, then 1 mg monthly maintenance.
- **B12 deficiency without severe neurologic signs**: 1 mg IM/SC monthly, or 1–2 mg oral daily (if absorption intact).
- **Peripheral neuropathy**: 1–1.5 mg oral/day divided, or 500 mcg–1 mg IM/SC 2–3 times weekly.
- **Maximum injection**: Often 1 mg per dose; higher doses (up to 2 mg) used in some protocols, but evidence limited.
## Pediatric Dosing
- **B12 deficiency (all ages)**: 1 mg IM/SC daily for 1 week, then 1 mg weekly for 4 weeks, then 1 mg monthly. For mild deficiency: 100–1000 mcg IM/SC daily for 1 week, then weekly for 1 month.
- **Congenital B12 malabsorption (e.g., Imerslund-Gräsbeck)**: 1 mg IM/SC monthly lifelong.
- **Oral dosing for maintenance**: 1–5 mg daily oral (liquid or chewable) if absorption normal; monitor levels.
- **Maximum pediatric injection**: 1 mg per dose (in older children). Neonates: typically 50–100 mcg IM daily for initial therapy, then adjust.
## Dose Adjustments
- **Renal impairment**: No specific adjustment needed; caution with high doses in severe renal failure (potential cyanide accumulation with other B12 forms, but methylcobalamin is safer).
- **Hepatic impairment**: No adjustment.
- **Elderly**: Same as adult; consider longer duration for deficiency correction.
## Contraindications
- Hypersensitivity to methylcobalamin or any component.
- Leber's disease (optic atrophy) — may accelerate optic nerve damage (relative).
- Caution in severe hypokalemia or polycythemia vera (due to rapid cell proliferation with B12 therapy).
## Adverse Effects
- **Common**: Injection site pain, mild diarrhea, nausea.
- **Rare**: Hypokalemia (with rapid RBC production), peripheral vascular thrombosis, anaphylaxis, urticaria.
- **High oral doses can cause**: Headache, dizziness, acneiform rash, peripheral edema (uncommon).
## Key Drug Interactions
- **Chloramphenicol**: May impair erythropoietic response to B12.
- **Omeprazole, H2 blockers, metformin**: Long-term use can reduce B12 absorption; monitor levels.
- **Colchicine, neomycin, alcohol**: Decrease B12 absorption.
- **Potassium supplements**: May reduce B12 absorption.
## Monitoring
- Baseline: Vitamin B12, homocysteine, methylmalonic acid (MMA), CBC with MCV, reticulocyte count, serum potassium.
- During therapy: Monitor B12, MMA, homocysteine every 1–3 months until normal; then annually. Check potassium early in treatment (first 48 hours) in severe anemia/high cell turnover.
- Clinical signs: Neurologic improvement (paresthesia, gait, cognition).
## Clinical Pearls
- Methylcobalamin is preferred over cyanocobalamin for neurologic conditions due to higher CNS bioavailability.
- Oral doses of 1–2 mg daily can be effective for deficiency even with mild malabsorption; sublingual may enhance absorption.
- Injections should be given IM (deep) or SC; avoid IV route (not standard).
- In vegan/vegetarian patients, consider low-dose oral maintenance (500–1000 mcg/day) if deficiency risk persists.
- When using high-dose therapy for neuropathy, clinical response may take weeks; reassess after 3 months.
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*This information is educational and not a substitute for professional judgment. Dosing may vary by local protocol and formulation. Always verify current prescribing information, contraindications, and monitoring requirements from the latest drug monograph or official guidelines.*
