Introduction
Methocarbamol is a centrally-acting skeletal muscle relaxant approved by the FDA in 1957. It is commonly prescribed as an adjunct to rest, physical therapy, and other measures for the relief of discomfort associated with acute, painful musculoskeletal conditions. Available in both oral and injectable formulations, methocarbamol works by depressing central nervous system activity rather than directly affecting skeletal muscle fibers.
Mechanism of Action
Methocarbamol's exact mechanism of action is not fully understood. It is believed to exert its effects through central nervous system depression, particularly by inhibiting nerve transmission in polysynaptic pathways in the brainstem and spinal cord. The drug does not directly relax tense skeletal muscles but appears to modify central perception of pain without altering pain threshold. Methocarbamol may also possess mild sedative properties that contribute to its muscle relaxant effects.
Indications
- FDA-approved: Relief of discomfort associated with acute, painful musculoskeletal conditions
- Common off-label uses: Muscle spasm management in various conditions including back pain, tetanus, and post-operative care
- Injectable form: Adjunctive management of tetanus when neuromuscular blockade is required
Dosage and Administration
Oral administration:- Adults: 1,500 mg four times daily initially; may be reduced to 750 mg four times daily or 1,500 mg three times daily after initial response
- Maximum daily dose: 8 grams for no more than 2-3 consecutive days
- Adults: 1-3 g daily in divided doses, not to exceed 3 g daily for more than 3 consecutive days
- Tetanus: Up to 24 g daily may be required
- Renal impairment: Use with caution; consider dose reduction
- Hepatic impairment: Use with caution
- Geriatric patients: Initiate at lower end of dosing range
- Pediatric patients: Safety and efficacy not established
Pharmacokinetics
- Absorption: Well absorbed from gastrointestinal tract; oral bioavailability approximately 90%
- Distribution: Widely distributed throughout body tissues; crosses placenta and enters breast milk
- Metabolism: Extensively metabolized in liver via dealkylation and hydroxylation
- Elimination: Primarily renal excretion (80-90%) as metabolites; elimination half-life approximately 1-2 hours
- Onset of action: Oral: 30 minutes; IV: immediate
Contraindications
- Hypersensitivity to methocarbamol or any component of the formulation
- Renal impairment (for injectable form due to polyethylene glycol content)
- Concomitant use with alcohol or other CNS depressants (relative contraindication)
Warnings and Precautions
- CNS depression: May impair mental and/or physical abilities required for hazardous tasks
- Seizures: Has been associated with seizures in overdose situations
- Pregnancy: Category C - use only if potential benefit justifies potential risk
- Lactation: Excreted in breast milk; use with caution
- Renal impairment: Use caution with injectable form due to vehicle (polyethylene glycol)
- Dependence: Psychological dependence uncommon but possible with prolonged use
Drug Interactions
- CNS depressants: Enhanced sedative effects with alcohol, benzodiazepines, opioids, sedating antihistamines, and other CNS depressants
- MAO inhibitors: Potential for enhanced CNS effects
- Pyridostigmine: May antagonize effects in myasthenia gravis
- Anticholinergics: Additive effects possible
Adverse Effects
Common (≥1%):- Dizziness
- Drowsiness
- Headache
- Nausea
- Blurred vision
- Fever
- Rash
- Urticaria
- Pruritus
- Flushing
- Hypotension
- Bradycardia
- Syncope
- Anaphylactic reaction
- Seizures (particularly with overdose)
- Severe allergic reactions
- Leukopenia (rare)
Monitoring Parameters
- Efficacy: Reduction in muscle spasm and pain
- Adverse effects: Sedation, dizziness, gastrointestinal symptoms
- Renal function (with prolonged therapy or in patients with renal impairment)
- Mental status changes
- Signs of allergic reaction
Patient Education
- Take exactly as prescribed; do not exceed recommended dosage
- Avoid alcohol and other CNS depressants during therapy
- May cause drowsiness or dizziness - use caution when driving or operating machinery
- Tablets may be crushed and mixed with food or liquid if swallowing is difficult
- Report any signs of allergic reaction (rash, itching, swelling)
- Inform all healthcare providers of methocarbamol use
- Do not abruptly discontinue after long-term use without medical supervision
- Store at room temperature away from moisture and heat
References
1. FDA Prescribing Information: Robaxin (methocarbamol) tablets and injection 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: a review of carisoprodol, cyclobenzaprine hydrochloride, diazepam, and methocarbamol. Clin Ther. 2004;26(9):1355-1367. 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: American Society of Health-System Pharmacists; 2023.