Generic Name

Robaxin

Mechanism

• Central nervous system depressant that reduces polysynaptic reflex activity in the spinal cord.
• Likely enhances inhibitory gamma‑aminobutyric acid (GABA) neurotransmission and/or modulates other excitatory pathways, resulting in decreased motoneuron excitability.
• Does not act directly on skeletal muscle fibers; rather, it lowers the neural drive to muscles.

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Pharmacokinetics

• Absorption: Oral bioavailability ~90 %; peak plasma concentrations 2–4 h post‑dose.
• Distribution: Lipophilic; extensive tissue distribution.
• Metabolism: Hepatic conversion to inactive metabolites (mainly via glucuronidation).
• Excretion: Renal (≈ 60 % unchanged drug) and hepatic.
• Half‑life: 2.5–5 h (dependent on renal/hepatic function).
• Drug interactions: May potentiate CNS depression when combined with alcohol, benzodiazepines, or opioids.

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Indications

• Acute musculoskeletal spasm (e.g., low back pain, neck strain).
• Post‑operative pain from minor procedures.
• Whiplash-associated disorders and other short‑term spinal injuries.

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Contraindications

• Hypersensitivity to methocarbamol or any formulation component.
• Pregnancy (Category C; caution advised).
• Liver or renal impairment: dose adjustment or avoidance.
• Severe CNS depression (e.g., coma, severe sleep apnea).
• Caution in patients on other CNS depressants or anticholinergic drugs.

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Dosing

• Administration: Oral tablets or IV infusion (for acute inpatient settings).
• Avoid mixing with benzodiazepines or alcohol without monitoring.

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Adverse Effects

• Common (≈ 10 %): dizziness, sedation, nausea, vomiting, anorexia, somnolence.
• Serious: severe allergic reactions (anaphylaxis), respiratory depression in high doses, hepatotoxicity (rare).
• Note: Rare reports of mild headaches, fatigue, and visual disturbances.

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Monitoring

• Baseline: CBC, CMP (ALT/AST, BUN/Cr).
• Follow‑up: Repeat CMP in > 7 days if prolonged therapy planned, monitor liver enzymes.
• Clinical: Assess for CNS depression, pain reduction, and any signs of intolerance or allergy.

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Clinical Pearls

• “First‑line in the emergency department”: Methocarbamol is FDA‑approved for acute spasms but operates slower than local anesthetics; use in mild to moderate pain before resorting to opioids.
• Drug interaction vigilance: Combining methocarbamol with benzodiazepines, opioids, or alcohol may precipitate deep sedation or respiratory depression—emphasize on discharge meds.
• Renal/hepatic dosing: A 50 % dose reduction is recommended for CrCl < 30 mL/min; no dosing recommendation exists for severe hepatic failure—use with caution.
• Pregnancy: Category C; limited data but no teratogenic findings—use only if benefits outweigh potential risks.
• Pediatric use: Not recommended under 12 yrs due to insufficient safety data; adult dosing may be extrapolated cautiously if necessary.

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• Key take-home: Methocarbamol (Robaxin) is a useful, centrally acting muscle relaxant for short‑term spasm control, but demands careful patient selection, monitoring, and awareness of additive CNS depressant effects.