Generic Name

Methocarbamol

Mechanism

• Central nervous system (CNS) depressant – acts on brain‑stem reflex centers that regulate muscle tone, reducing alpha‑motor neuron excitability.
• Increases glycine transmission and potentiates GABA‑mediated chloride influx, ultimately dampening neuronal firing.
• It does not work directly on skeletal muscle or NMJ; all effects are centrally mediated.

Pharmacokinetics

• Absorption: Oral bioavailability ~25 %; peak serum concentrations in 1–4 h.
• Distribution: Widely distributed; protein binding ~50 %.
• Metabolism: Primarily hepatic via glucuronidation.
• Elimination: Renal excretion of unchanged drug and metabolites.
• Half‑life: 2–6 h in healthy adults; prolonged to ~3–7 h in elderly or hepatic dysfunction.

Indications

• *Reversible muscle spasm* associated with acute musculoskeletal injury (e.g., sprain, strain).
• Often combined with analgesics for spasm‑related pain management.
• Adjunct in multimodal pain protocols (e.g., spinal cord injuries, thoracic outlet syndrome).

Contraindications

• Allergy to methocarbamol or IPC (isopropyl carbamate) derivatives.
• Bacterial meningitis (due to potential CNS toxicity).
• Pregnancy category C – use only if benefits outweigh risks.
• Use caution in patients:
• Severe hepatic impairment
• Renal disease
• Elderly (increased CNS depression risk)
• Alcohol abuse or CNS depressants (additive sedation)

> Warning: May reverse neuroprotective effects of hypothermia; avoid in emergent hypothermic care.

Dosing

| Oral | 500 mg every 6 h (maximum 2000 mg/day) | 3–4 ×/day | 10–70 kg children: 6.8 mg/kg bid; Admin note: Premedication with antihistamine may reduce infusion‑related flushing.

Adverse Effects

• Common: nausea, vomiting, dizziness, visual disturbances, mild sedation, hypotonia.
• Serious: respiratory depression (rare), severe CNS depression, hypotension, hepatic enzyme elevations, anaphylactoid reactions, skin rash.
• Drug interactions: additive CNS depression with alcohol, benzodiazepines, opioids, barbiturates.

Monitoring

• Observe for:
• CNS depression – respiratory rate, level of consciousness.
• Hepatic function – ALT/AST every 2–4 weeks during prolonged therapy.
• Renal function – BUN/Cr baseline, then quarterly.
• Signs of allergic reaction – rash, pruritus, hypotension.
• Consider urinary output if constipation or urinary retention is reported.

Clinical Pearls

• Peripheral edema can appear; advise patient to elevate legs if swelling develops.
• Drug cross‑reaction: Patients allergic to cyclobenzaprine may tolerate methocarbamol, except for shared metabolite‑based adverse effects.
• Taper strategy: Discontinue after 1–2 days; if extended use required, taper by 25 % every 3 days to mitigate rebound spasm.
• IV infusion: Rapid IV administration (within 30 s) can precipitate flushing; dilute and infuse over ≥5 min.
• Pregnancy & lactation: Cross placental barrier; minimal data – use only when essential.
• Age‑related sensitivity: Elderly patients may experience prolonged half‑life; begin at one‑third of standard dose if there are CNS risk factors.

Key Takeaway: Methocarbamol is a centrally acting, first‑line muscle relaxant for acute musculoskeletal spasm, best used short‑term, with meticulous dose adjustment for special populations and vigilance for CNS depression and hepatic dysfunction.