ROBAXIN-750
Clinical safety rating
cautionComprehensive 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. |
| Molecular Weight | 241.24 |
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 | Animal studies show fetal risk; no adequate human studies. Use only if benefit outweighs risk. |
| 2nd trimester | Animal studies show fetal risk; no adequate human studies. Use only if benefit outweighs risk. |
| 3rd trimester | Avoid due to risk of neonatal hemorrhage and withdrawal symptoms. |
Clinical note
Comprehensive clinical and safety monograph for ROBAXIN-750 (ROBAXIN-750).
| Placental transfer | Crosses the placenta; levels in fetal circulation are lower than maternal. |
| Breastfeeding | Not recommended during breastfeeding because methocarbamol is excreted in breast milk and may cause sedation or other adverse effects in the infant. |
| Lactation Rating | L3 (Moderately Safe) |
| 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. |
| 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. |
■ FDA Black Box Warning
None
| Serious Effects |
Hypersensitivity to methocarbamolRenal impairment (severe)Hepatic impairment (severe)Myasthenia gravisConcurrent use of CNS depressants (relative)
| 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. |
| Food/Dietary | Take with food to reduce gastrointestinal upset. Avoid alcohol during treatment. No specific food restrictions. |
| Clinical Pearls | ROBAXIN-750 (methocarbamol 750 mg) is a centrally acting muscle relaxant. It does not directly relax skeletal muscle but acts via CNS depression. Onset of action is ~30 minutes; peak effect at 2 hours. It is often used with rest, physical therapy, and NSAIDs. Contraindicated in myasthenia gravis. Doses exceeding 6 g/day may cause severe adverse effects. Tablet contains FD&C Yellow No. 5 (tartrazine), which may cause allergic reactions in susceptible individuals. |
| Patient Advice | Take exactly as prescribed; do not increase dose or frequency without consulting your doctor. · This medication may cause drowsiness, dizziness, or blurred vision; avoid driving or operating machinery until you know how it affects you. · Avoid alcohol and other CNS depressants (e.g., sedatives, tranquilizers) as they may increase sedation. · Notify your doctor if you have myasthenia gravis, allergies to tartrazine (FD&C Yellow No. 5), or if you are pregnant or breastfeeding. · May discolor urine (brown, black, or blue-green); this is harmless. · Do not stop suddenly; withdrawal symptoms (e.g., anxiety, insomnia) may occur with abrupt discontinuation after prolonged use. |
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