METHOCARBAMOL

Clinical safety rating: safe

Animal studies have demonstrated safety

How it works

Methocarbamol is a centrally acting muscle relaxant whose exact mechanism of action is not fully understood. It is thought to produce skeletal muscle relaxation by depressing the central nervous system, possibly via general CNS depression, without directly affecting the neuromuscular junction or skeletal muscle fibers.

What the body does with it

Metabolism	Metabolized by the liver via dealkylation and hydroxylation. The major metabolic pathway involves O-dealkylation to form a glycinate conjugate, with CYP450 enzymes likely involved.
Excretion	Renal: primarily as glucuronide conjugates and unchanged drug (~50-70% as metabolites, <2% unchanged). Fecal: minimal, <2%. Biliary: not significant.
Half-life	Terminal elimination half-life: 1-2 hours. Clinical context: short half-life necessitates frequent dosing (q6h) for sustained muscle relaxation.
Protein binding	Protein binding: 46-50% to albumin.
Volume of Distribution	Volume of distribution: 0.6-0.8 L/kg. Clinical meaning: distributes widely into tissues, moderate Vd indicating extravascular distribution.
Bioavailability	Oral: high bioavailability, ~80-100% (well absorbed with first-pass metabolism to inactive conjugates). Intravenous: 100%.
Onset of Action	Oral: onset within 30 minutes. Intravenous: immediate (within minutes).
Duration of Action	Oral: 4-6 hours. Intravenous: 2-4 hours. Clinical notes: effects are dose-dependent and shorter with higher clearance.

Classification & Brands

Dosing & administration

METHOCARBAMOL 1500 mg orally 4 times daily or 750 mg orally every 4 hours, or 1-3 g intravenously every 8 hours, not to exceed 3 g/day intravenously for more than 3 consecutive days.

Dosage form	TABLET
Renal impairment	CrCl <50 mL/min: Administer every 8-12 hours; CrCl <30 mL/min: Administer every 12 hours; hemodialysis: Supplementation not well-defined; avoid if possible due to propylene glycol content.
Liver impairment	Child-Pugh A: No adjustment; Child-Pugh B: Reduce dose by 50%; Child-Pugh C: Contraindicated.
Pediatric use	Not recommended for children under 16 years; safety and efficacy not established.
Geriatric use	Start at lower end of dosing range (e.g., 750 mg orally 4 times daily) due to increased risk of sedation and falls; monitor renal function and adjust accordingly.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

CNS depressants may enhance sedative effects Can cause discoloration of urine.

Breastfeeding	Excreted in breast milk in small amounts; M/P ratio not established. No reported adverse effects in infants. Caution is advised due to potential for CNS depression or muscle weakness.
Teratogenic Risk	FDA Pregnancy Category C. First trimester: Animal studies show fetal abnormalities (reduced fetal weight, skeletal ossification delays) at doses 1-3 times human dose; no adequate human studies. Second and third trimesters: Potential for neonatal CNS depression and hypotonia if used near term. Avoid use unless benefit outweighs risk.

Warnings & precautions

■ FDA Black Box Warning

No FDA black box warning.

Side Effect Profile

Common Effects	Drowsiness
Serious Effects

Absolute Contraindications

Hypersensitivity to methocarbamol or any component of the formulation; concomitant use of anticholinesterase drugs in patients with myasthenia gravis (contraindicated); known history of G6PD deficiency (relative, due to risk of hemolytic anemia).

Clinical Precautions

Precautions

May cause dizziness, drowsiness, or blurred vision; caution with activities requiring mental alertness. Use with caution in patients with hepatic impairment, renal impairment, or myasthenia gravis. Avoid concurrent use with other CNS depressants. May cause urine discoloration (brown, black, or blue).

Clinical Tips & Counseling

Drug interactions

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