ROBAXISAL
Clinical safety rating
cautionComprehensive clinical and safety monograph for ROBAXISAL (ROBAXISAL).
Methocarbamol is a centrally acting muscle relaxant whose exact mechanism is not fully understood, but it is believed to involve general central nervous system depression and inhibition of polysynaptic reflexes in the spinal cord. Aspirin inhibits cyclooxygenase enzymes (COX-1 and COX-2), reducing prostaglandin synthesis, which provides analgesic and anti-inflammatory effects, and also irreversibly inhibits platelet aggregation.
| Metabolism | Methocarbamol is metabolized primarily via dealkylation and glucuronidation; unknown cytochrome P450 involvement. Aspirin is hydrolyzed to salicylic acid, which is then conjugated with glycine (forming salicyluric acid) and glucuronic acid, and undergoes oxidative metabolism via CYP2C9 (minor pathway). |
| Excretion | Methocarbamol: renal (primarily as glucuronide and sulfate conjugates, with <2% unchanged); guaifenesin: renal (metabolites, <1% unchanged). No significant biliary/fecal elimination. |
| Half-life | Methocarbamol: 1.0–2.0 hours (prolonged in renal impairment); guaifenesin: approximately 1 hour. |
| Protein binding | Methocarbamol: 46–50% (primarily to albumin); guaifenesin: negligible. |
| Volume of Distribution | Methocarbamol: 0.6–0.8 L/kg; guaifenesin: not well characterized (likely <1 L/kg). |
| Bioavailability | Methocarbamol: ~100% oral (well absorbed, but extensive first-pass metabolism); guaifenesin: ~80–90% oral (undergoes first-pass metabolism). |
| Onset of Action | Oral methocarbamol: approximately 30 minutes; oral guaifenesin: 30–60 minutes. |
| Duration of Action | Methocarbamol: 4–6 hours (relaxant effect); guaifenesin: 4–6 hours (expectorant effect). |
| Molecular Weight | Methocarbamol: 241.24 Da; Aspirin: 180.16 Da |
Oral: 2 tablets (methocarbamol 750 mg / aspirin 650 mg) 4 times daily.
| Dosage form | TABLET |
| Renal impairment | Contraindicated if GFR <30 mL/min due to aspirin component. For GFR 30-60 mL/min, reduce dose to 1 tablet 4 times daily; avoid if possible. |
| Liver impairment | Child-Pugh A: No adjustment needed. Child-Pugh B or C: Avoid use due to aspirin component (risk of bleeding) and methocarbamol metabolism concerns. |
| Pediatric use | Not recommended for use in children and adolescents due to risk of Reye's syndrome from aspirin. |
| Geriatric use | Avoid use in elderly due to increased risk of gastrointestinal bleeding, renal impairment, and drug interactions; consider alternative therapy. |
| 1st trimester | Avoid; contains methocarbamol (pregnancy category C) and aspirin (category D in third trimester, but associated with cardiovascular and CNS defects in first trimester). |
| 2nd trimester | Avoid; aspirin may cause oligohydramnios and constriction of ductus arteriosus. |
| 3rd trimester | Contraindicated; aspirin increases risk of premature closure of ductus arteriosus, prolonged labor, and maternal/neonatal bleeding. |
Clinical note
Comprehensive clinical and safety monograph for ROBAXISAL (ROBAXISAL).
| Placental transfer | Both components cross the placenta: aspirin readily crosses; methocarbamol crosses in animals but human data limited. |
| Breastfeeding | Aspirin is excreted into breast milk and may cause Reye syndrome, metabolic acidosis, and platelet dysfunction in infants. Methocarbamol excretion is minimal but safety not established. Use not recommended. |
| Lactation Rating | L5 (Contraindicated) |
| Teratogenic Risk | First trimester: No well-controlled studies; animal data suggest risk of skeletal anomalies at high doses; avoid unless benefit outweighs risk. Second/third trimester: No known teratogenicity; preterm labor risk with high doses due to skeletal muscle relaxation. |
| Fetal Monitoring | Monitor fetal heart rate and uterine activity with intravenous high-dose methocarbamol. Assess bleeding time in mother due to aspirin component. Observe neonate for respiratory depression if used near delivery. |
| Fertility Effects | No specific data on human fertility; animal studies show no significant effects. Aspirin may impair ovulation at high doses due to prostaglandin inhibition. |
■ FDA Black Box Warning
No FDA black box warning is present for ROBAXISAL.
| Serious Effects |
Hypersensitivity to methocarbamol, aspirin, or NSAIDsThird trimester pregnancyLactationActive peptic ulcer diseaseHemophilia or bleeding disordersSevere hepatic impairmentSevere renal impairmentHistory of aspirin-exacerbated respiratory diseaseChildren with viral infections (Reye syndrome risk)
| Precautions | Risk of Reye syndrome with aspirin use in children or teenagers with viral illnesses, Methocarbamol may cause CNS depression (drowsiness, dizziness) and impair mental or physical abilities, Aspirin increases risk of bleeding, including gastrointestinal bleeding and intracranial hemorrhage, Methocarbamol may cause seizures in patients with known seizure disorders, Cross-allergenicity with aspirin in patients with NSAID hypersensitivity |
| Food/Dietary | Avoid alcohol and caffeine-containing beverages. Take with food to minimize GI irritation. Do not take with other NSAIDs, aspirin, or anticoagulants. Avoid high-fat meals, which may delay absorption of aspirin. |
| Clinical Pearls | ROBAXISAL combines methocarbamol, a centrally acting muscle relaxant, with aspirin, an NSAID. Monitor for GI bleeding, especially in elderly or those on anticoagulants. Avoid in patients with peptic ulcer disease or aspirin allergy. Methocarbamol may cause sedation and impair motor skills; caution with driving or operating machinery. Onset of methocarbamol's muscle relaxant effect is within 30 minutes; aspirin absorption is rapid. Not recommended for use beyond 2-3 weeks due to lack of long-term efficacy data. |
| Patient Advice | Take with food or a full glass of water to reduce stomach upset. · Do not crush or chew the tablets; swallow whole. · Avoid alcohol, as it increases sedation and GI irritation risk. · Do not use other aspirin-containing or NSAID products while taking this medication. · Report signs of bleeding (black stools, vomit with coffee-ground appearance) or allergic reactions (rash, swelling, difficulty breathing) immediately. · This medication may cause drowsiness or dizziness; avoid driving or hazardous activities until you know how it affects you. · Do not use for more than 2-3 weeks unless directed by your doctor. |
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