ROBAXIN

Clinical safety rating: caution

Comprehensive clinical and safety monograph for ROBAXIN (ROBAXIN).

How it works

Centrally acting muscle relaxant; depresses polysynaptic reflexes at spinal cord and supraspinal levels, possibly via glycine receptor agonism and GABAergic modulation.

What the body does with it

Metabolism	Hepatic metabolism via CYP450 enzymes (primarily CYP1A2, CYP3A4); active metabolite (methocarbamol).
Excretion	Renal excretion of metabolites accounts for 99% of elimination; <1% excreted as unchanged drug in urine.
Half-life	1-2 hours in adults; clinically, multiple daily dosing required to maintain effect.
Protein binding	50% bound to albumin.
Volume of Distribution	0.5-1.0 L/kg; indicates distribution into total body water.
Bioavailability	Oral: 80-100% (extensive absorption, first-pass metabolism minimal).
Onset of Action	Oral: 30 minutes; IV: immediate (within minutes).
Duration of Action	Oral: 4-6 hours; IV: 1-2 hours. Muscle relaxation persists up to 6 hours after oral dose.

Classification & Brands

Dosing & administration

1500 mg orally 4 times daily, or 750 mg orally every 4 hours as needed. Maximum 6 g/day. For IV use: 1 g (10 mL) as a single intravenous injection or infusion.

Dosage form	TABLET
Renal impairment	In severe renal impairment (CrCl <30 mL/min), reduce dose by 50% and monitor for CNS effects. Avoid use in anuria.
Liver impairment	Contraindicated in severe hepatic impairment (Child-Pugh class C). Use with caution in mild to moderate impairment; reduce dose by 50% and monitor for sedation.
Pediatric use	Not recommended for children under 12 years. For ages ≥12 years: same as adult dosing.
Geriatric use	Initiate at reduced dose (e.g., 750 mg 4 times daily) due to increased sensitivity and risk of sedation; titrate cautiously and monitor renal function.

Use during pregnancy

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

Clinical note

Comprehensive clinical and safety monograph for ROBAXIN (ROBAXIN).

Breastfeeding	Methocarbamol is excreted in human milk in small amounts. The M/P ratio is unknown. Caution should be exercised when administered to a nursing woman. Monitor infant for sedation.
Teratogenic Risk	No adequate well-controlled studies in pregnant women. Animal reproduction studies have not been conducted. Risk cannot be ruled out. Use only if clearly needed. First trimester: potential risk unknown. Second and third trimesters: may be associated with neonatal respiratory depression if used near delivery.
Fetal Monitoring

Warnings & precautions

■ FDA Black Box Warning

None

Side Effect Profile

Serious Effects

Absolute Contraindications

["Hypersensitivity to methocarbamol or any component","Myasthenia gravis","Known hypersensitivity to cyclobenzaprine or related compounds (if formulation includes)"]

Clinical Precautions

Precautions

["May cause drowsiness or dizziness; caution with driving or operating machinery","Risk of serotonin syndrome with concomitant serotonergic drugs","Hepatic impairment; monitor liver function","Renal impairment; caution in severe cases","Use with caution in elderly and debilitated patients"]

Clinical Tips & Counseling

Drug interactions

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ROBAXIN

Clinical safety rating: caution

Comprehensive clinical and safety monograph for ROBAXIN (ROBAXIN).

How it works

Centrally acting muscle relaxant; depresses polysynaptic reflexes at spinal cord and supraspinal levels, possibly via glycine receptor agonism and GABAergic modulation.

What the body does with it

Metabolism	Hepatic metabolism via CYP450 enzymes (primarily CYP1A2, CYP3A4); active metabolite (methocarbamol).
Excretion	Renal excretion of metabolites accounts for 99% of elimination; <1% excreted as unchanged drug in urine.
Half-life	1-2 hours in adults; clinically, multiple daily dosing required to maintain effect.
Protein binding	50% bound to albumin.
Volume of Distribution	0.5-1.0 L/kg; indicates distribution into total body water.
Bioavailability	Oral: 80-100% (extensive absorption, first-pass metabolism minimal).
Onset of Action	Oral: 30 minutes; IV: immediate (within minutes).
Duration of Action	Oral: 4-6 hours; IV: 1-2 hours. Muscle relaxation persists up to 6 hours after oral dose.

Classification & Brands

Dosing & administration

1500 mg orally 4 times daily, or 750 mg orally every 4 hours as needed. Maximum 6 g/day. For IV use: 1 g (10 mL) as a single intravenous injection or infusion.

Dosage form	TABLET
Renal impairment	In severe renal impairment (CrCl <30 mL/min), reduce dose by 50% and monitor for CNS effects. Avoid use in anuria.
Liver impairment	Contraindicated in severe hepatic impairment (Child-Pugh class C). Use with caution in mild to moderate impairment; reduce dose by 50% and monitor for sedation.
Pediatric use	Not recommended for children under 12 years. For ages ≥12 years: same as adult dosing.
Geriatric use	Initiate at reduced dose (e.g., 750 mg 4 times daily) due to increased sensitivity and risk of sedation; titrate cautiously and monitor renal function.

Use during pregnancy

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

Clinical note

Comprehensive clinical and safety monograph for ROBAXIN (ROBAXIN).

Breastfeeding	Methocarbamol is excreted in human milk in small amounts. The M/P ratio is unknown. Caution should be exercised when administered to a nursing woman. Monitor infant for sedation.
Teratogenic Risk	No adequate well-controlled studies in pregnant women. Animal reproduction studies have not been conducted. Risk cannot be ruled out. Use only if clearly needed. First trimester: potential risk unknown. Second and third trimesters: may be associated with neonatal respiratory depression if used near delivery.
Fetal Monitoring

Warnings & precautions

■ FDA Black Box Warning

None

Side Effect Profile

Serious Effects

Absolute Contraindications

["Hypersensitivity to methocarbamol or any component","Myasthenia gravis","Known hypersensitivity to cyclobenzaprine or related compounds (if formulation includes)"]