Methocarbamol is a commonly used, centrally acting muscle relaxant and has not been linked to instances of liver injury.
Methocarbamol is a guaifenesin derivative and acts centrally as a muscle relaxant by an unknown mechanism. Methocarbamol was approved for use in the United States in 1957, and currently more than 3 million prescriptions are filled yearly. Methocarbamol is indicated for the relief of acute, painful musculoskeletal conditions. Methocarbamol is available in 500 and 750 mg tablets in several generic formulations both alone and in combination with other drugs and under the brand names of Robaxin and Marbaxin. The recommended dosage is 1500 mg orally three to four times daily. The most common side effects of methocarbamol are drowsiness blurred vision, headache, nausea and skin rash.
While the product label for methocarbamol states that it can cause jaundice (including cholestatic jaundice), there is little published evidence to suggest that methocarbamol is a cause of hepatic injury or clinically apparent drug induced liver disease. During clinical trials of methocarbamol, some patients had to stop treatment because of nausea, dizziness, or other non-specific complaints, but no serum aminotransferase levels or other laboratory results were reported. Methocarbamol appears to be well tolerated, but the lack of monitoring of serum aminotransferase levels during clinical trials with methocarbamol makes it impossible to rule out the possibility of mild liver injury occurring with treatment.
Drug Class: Muscle Relaxant Drugs
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Zimmerman HJ. Muscle spasmolytics. In, Hepatotoxicity: The Adverse Effects of Drugs and Other Chemicals on the Liver. 2nd Ed. Philadelphia: Lippincott, 1999. p. 544-45. (Expert review of hepatotoxicity published in 1999; discussed dantrolene, chlorzoxazone and baclofen but not methocaramol).
Taylor P. Agents acting at the neuromuscular junction and autonomic ganglia. In, Brunton LL, Lazo JS, Parker KL, eds. Goodman & Gilman’s The pharmacological basis of therapeutics, 11th ed. New York: McGraw-Hill, 2006. p. 217-36. (Textbook of Pharmacology and Therapeutics).
Chou R, Peterson K, Helfand M. Comparative efficacy and safety of skeletal muscle relaxants for spasticity and musculoskeletal conditions: a systematic review. J Pain Symptom Manage 2004; 28: 140-75. PubMed Citation (Thorough review of the pharmacology, efficacy and side effects of the muscle relaxants)
Chalasani N, Fontana RJ, Bonkovsky HL, Watkins PB, Davern T, Serrano J, Yang H, Rochon J; Drug Induced Liver Injury Network (DILIN). Causes, clinical features, and outcomes from a prospective study of drug-induced liver injury in the United States. Gastroenterology 2008; 135: 1924-34. PubMed Citation. (Among 300 cases of drug-induced liver disease in the U.S. collected from 2004 to 2008, one was attributed to chlorzoxazone but none to methocarbamol).