ROBAXIN-750
Clinical safety rating: caution
Comprehensive clinical and safety monograph for ROBAXIN-750 (ROBAXIN-750).
Methocarbamol, the active ingredient in Robaxin-750, is a centrally acting muscle relaxant. Its precise mechanism is not fully understood, but it is believed to cause general central nervous system depression, possibly through inhibition of polysynaptic reflexes at the spinal cord level.
| Metabolism | Hepatic metabolism via dealkylation and hydroxylation, primarily by cytochrome P450 enzymes; metabolites are excreted renally. |
| Excretion | Renal: 90-95% as metabolites (mainly conjugated), <1% unchanged; biliary/fecal: minor; <2% eliminated in feces. |
| Half-life | Terminal elimination half-life: 1-2 hours (methocarbamol); clinical context: short half-life necessitates frequent dosing (q6h) and may lead to fluctuating plasma levels. |
| Protein binding | 46-50% (primarily to albumin); binding is reversible and non-saturable at therapeutic concentrations. |
| Volume of Distribution | 0.9-1.5 L/kg (apparent Vd); clinical meaning: indicates extensive distribution into total body water and tissues, including muscle. |
| Bioavailability | Oral: about 50-75% (due to first-pass metabolism); bioavailability may vary with food (high-fat meal reduces rate but not extent). |
| Onset of Action | Oral: 30 minutes (skeletal muscle relaxation); peak effect: 1-2 hours. |
| Duration of Action | Oral: 4-6 hours; clinical note: effects may persist slightly longer due to active metabolites, but dosing interval is typically every 6 hours. |
750 mg orally four times daily (total daily dose 3000 mg).
| Dosage form | TABLET |
| Renal impairment | GFR 30-50 mL/min: administer 750 mg every 8 hours; GFR <30 mL/min: avoid use or administer with extreme caution; not studied in dialysis. |
| Liver impairment | Child-Pugh Class A: no adjustment; Child-Pugh Class B: reduce dose by 30-50% (e.g., 375-500 mg q6h); Child-Pugh Class C: contraindicated due to risk of hepatic encephalopathy. |
| Pediatric use | Not recommended for pediatric use; safety and efficacy not established. |
| Geriatric use | Initiate at low end of dosing range (e.g., 375 mg q6h) due to increased risk of sedation and falls; monitor renal function and CNS effects. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for ROBAXIN-750 (ROBAXIN-750).
| Breastfeeding | Methocarbamol is excreted into breast milk in small amounts; M/P ratio not established. The American Academy of Pediatrics considers it compatible with breastfeeding. Monitor infant for sedation or muscle weakness. |
| Teratogenic Risk | FDA Category C: Methocarbamol has shown fetal toxicity (reduced fetal weight, increased skeletal variations) in animal studies at doses similar to human doses. First trimester: Insufficient human data; avoid unless clearly needed. Second/third trimester: Limited data; potential for musculoskeletal effects. Use only if benefit outweighs risk. |
■ FDA Black Box Warning
None
| Serious Effects |
["Hypersensitivity to methocarbamol or any component of the formulation.","Patients with known or suspected myasthenia gravis.","Patients with renal impairment (due to propylene glycol content in the injectable form; oral formulation does not contain propylene glycol but should be used cautiously)."]
| Precautions | ["May cause drowsiness, dizziness, or blurred vision; patients should not operate machinery or drive until effects are known.","Use with caution in patients with known drug sensitivities or allergies.","May impair mental and/or physical abilities.","Seizures have been reported in patients with underlying seizure disorders or concurrent use of CNS depressants.","Serotonergic effects may occur with concurrent use of serotonergic drugs."] |
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| Fetal Monitoring | Monitor maternal blood pressure (risk of hypotension with IV use) and CNS effects (drowsiness, dizziness). Fetal monitoring not routinely required; assess fetal movement if maternal sedation significant. |
| Fertility Effects | No known adverse effects on fertility in animal studies. Human data lacking; theoretical impact due to CNS depression may affect libido or ovulation. Not recommended for fertility-related use. |