ROBAXIN
Clinical safety rating
cautionComprehensive clinical and safety monograph for ROBAXIN (ROBAXIN).
Centrally acting muscle relaxant; depresses polysynaptic reflexes at spinal cord and supraspinal levels, possibly via glycine receptor agonism and GABAergic modulation.
| 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. |
| Molecular Weight | 221.25 |
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. |
| 1st trimester | Risk not ruled out. Animal studies have shown adverse effects, but no adequate human studies. Use only if benefit outweighs risk. |
| 2nd trimester | Risk not ruled out. Use only if clearly needed. |
| 3rd trimester | Associated with neonatal respiratory depression and hypotonia if used near term. Avoid in late pregnancy, especially during labor. |
Clinical note
Comprehensive clinical and safety monograph for ROBAXIN (ROBAXIN).
| Placental transfer | Crosses the placenta; detected in cord blood after maternal administration. |
| Breastfeeding | Excreted into breast milk in low concentrations; however, because of the potential for serious adverse reactions in nursing infants, a decision should be made whether to discontinue nursing or discontinue the drug, taking into account the importance of the drug to the mother. |
| Lactation Rating | L3 (Moderately Safe) |
| 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 | Monitor maternal blood pressure and respiratory status. Fetal heart rate monitoring if used in third trimester or during labor. |
| Fertility Effects | No human data on fertility. Animal studies have not shown impaired fertility. |
■ FDA Black Box Warning
None
| Serious Effects |
Hypersensitivity to methocarbamol or any component of the formulationRenal impairment (anuria, severe renal disease)Myasthenia gravisConcurrent use with pyridostigmine or other anticholinesterase agents (theoretical)
| 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 |
| Food/Dietary | No significant food interactions. However, take with food if gastrointestinal upset occurs. Avoid alcohol. |
| Clinical Pearls | ROBAXIN (methocarbamol) is a centrally acting muscle relaxant indicated for acute musculoskeletal pain. Onset of action is within 30 minutes; peak effect at 2 hours. Avoid in patients with myasthenia gravis. Renal impairment requires dose adjustment. Can cause sedation; avoid concurrent CNS depressants. IV formulation may cause hemolysis if extravasation occurs. |
| Patient Advice | Drowsiness and dizziness are common; do not drive or operate machinery until you know how this medication affects you. · Avoid alcohol and other CNS depressants (e.g., benzodiazepines, opioids). · Take exactly as prescribed; do not increase dose or frequency. · May cause urine discoloration (brown, black, or blue-green); this is harmless. · Notify your doctor if you experience rash, itching, or difficulty breathing. · Do not stop abruptly; taper dose if discontinuing after prolonged use. |
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