Introduction
Robaxin (methocarbamol) is a centrally-acting skeletal muscle relaxant approved by the FDA for the relief of discomfort associated with acute, painful musculoskeletal conditions. It is used as an adjunct to rest, physical therapy, and other measures for the relief of muscle spasm.
Mechanism of Action
Methocarbamol exerts its effects through central nervous system depression. While its exact mechanism remains incompletely understood, it appears to act primarily by depressing polysynaptic reflexes in the spinal cord and potentially through sedative effects. Unlike other muscle relaxants, methocarbamol does not directly relax skeletal muscles.
Indications
- FDA-approved: Adjunctive treatment for acute musculoskeletal pain and spasm
- Off-label uses: May be used for muscle spasm associated with tetanus (requires higher doses)
Dosage and Administration
Adults:- Initial dose: 1,500 mg four times daily
- Maintenance: 4,000-4,500 mg daily in divided doses
- Maximum recommended dose: 6,000 mg daily for severe conditions
Pharmacokinetics
Absorption: Rapidly absorbed from GI tract Distribution: Widely distributed throughout body tissues Metabolism: Extensively metabolized in liver via dealkylation and hydroxylation Elimination: Primarily renal excretion (as metabolites) Half-life: Approximately 1-2 hours Onset of action: Within 30 minutes Duration of action: 4-6 hoursContraindications
- Hypersensitivity to methocarbamol or any component of the formulation
- Patients with impaired hepatic function
- Patients with renal impairment requiring dialysis
- History of allergic or idiosyncratic reactions to related compounds
Warnings and Precautions
- May impair mental and/or physical abilities required for performance of hazardous tasks
- Use with caution in patients with seizure disorders
- May cause dizziness, drowsiness, or blurred vision
- Potential for drug dependence with long-term use
- Discontinue if allergic reactions occur
- Use caution in patients with myasthenia gravis
Drug Interactions
CNS depressants: Enhanced sedative effects (alcohol, benzodiazepines, opioids) Anticholinergic agents: Additive effects MAO inhibitors: Potential for enhanced CNS depression Pyridostigmine: May antagonize effects in myasthenia gravisAdverse Effects
Common (≥1%):- Dizziness
- Drowsiness
- Nausea
- Headache
- Blurred vision
- Anaphylactic reactions
- Seizures
- Leukopenia
- Jaundice
- Syncope
Monitoring Parameters
- Therapeutic response and pain control
- CNS effects (sedation, dizziness)
- Liver function tests (periodically with long-term use)
- Renal function in elderly or compromised patients
- Signs of allergic reactions
- Complete blood count with prolonged therapy
Patient Education
- Take exactly as prescribed; do not exceed recommended dosage
- Avoid alcohol and other CNS depressants during therapy
- May cause drowsiness or dizziness; avoid driving or operating machinery until effects are known
- Report any signs of allergic reaction (rash, itching, swelling)
- Inform healthcare providers of all medications being taken
- Do not abruptly discontinue without medical supervision
- Store at room temperature away from moisture
References
1. FDA Prescribing Information: Robaxin (methocarbamol) tablets 2. Lexicomp Online: Methocarbamol monograph 3. Micromedex Solutions: Methocarbamol drug information 4. Toth PP, Urtis J. Commonly used muscle relaxant therapies for acute low back pain. Postgrad Med. 2014;126(1):139-148 5. See S, Ginzburg R. Choosing a skeletal muscle relaxant. Am Fam Physician. 2008;78(3):365-370 6. American Society of Health-System Pharmacists. AHFS Drug Information. Bethesda, MD