VA Class:MS200
Methocarbamol is a centrally acting skeletal muscle relaxant.
Methocarbamol is used as an adjunct to rest, physical therapy, analgesics, and other measures for the relief of discomfort associated with acute, painful musculoskeletal conditions.1, 2, 18, 110
Evidence supporting the efficacy of skeletal muscle relaxants is generally low to moderate in quality; while these agents appear to be more effective than placebo in providing short-term relief of acute low back pain, they are associated with a high incidence of adverse effects (e.g., sedation).103, 104, 106, 109, 110, 111, 112 Although comparative studies are limited, available data suggest that various skeletal muscle relaxants generally have similar efficacy for such use.103, 104, 106, 108 Acute low back pain usually is a benign and self-limiting condition that improves spontaneously over time;105, 106, 108, 109 therefore, nonpharmacologic treatment strategies (e.g., heat, massage) are recommended.109 If pharmacologic therapy is required, experts state that a nonsteroidal anti-inflammatory agent (NSAIA) or a skeletal muscle relaxant may be used; however, these drugs have been shown to result in only small improvements in pain relief and can increase the risk of adverse effects.104, 106, 107, 108, 109 In general, skeletal muscle relaxants should be used with caution after weighing the potential risks against the benefits in individual patients.104, 106, 107, 108 Although skeletal muscle relaxants are often used in combination with NSAIAs for the treatment of acute low back pain, randomized controlled studies generally have not demonstrated any additional improvement in pain or functional outcomes with such combination therapy compared with use of an NSAIA alone.110, 112, 113
It is unclear whether relief of musculoskeletal pain by methocarbamol results from skeletal muscle relaxant effects, sedative effects, or a placebo effect of the drug. Most authorities attribute the beneficial effects of skeletal muscle relaxants to their sedative properties.
Methocarbamol is ineffective in the treatment of skeletal muscle hyperactivity secondary to chronic neurologic disorders such as cerebral palsy and other dyskinesias.
Methocarbamol has been used as an adjunct to debridement, tetanus antitoxin, penicillin, tracheotomy, fluid and electrolyte replacement, and supportive therapy in the management of tetanus. However, most authorities prefer administration of diazepam, meprobamate, barbiturates, or chlorpromazine, and in severe cases, administration of neuromuscular blocking agents.
Although some clinicians have used IV or IM methocarbamol to terminate epileptic seizures†, the drug may precipitate seizures.2 (See Cautions: Adverse Effects.)
Methocarbamol is usually administered orally. When oral methocarbamol therapy is not feasible or in patients with severe musculoskeletal pain, the drug may be given IM or IV. Methocarbamol injection should not be administered subcutaneously.2
For IV administration, methocarbamol injection may be administered undiluted by direct IV injection (at a rate not exceeding 3 mL/minute), or may be diluted and administered by IV infusion.2 To prepare the solution for IV infusion, 1 g of methocarbamol may be diluted with up to 250 mL of 5% dextrose or 0.9% sodium chloride injection.2
The patient should be recumbent during and for at least 10-15 minutes following IV administration of methocarbamol.2 Extravasation should be avoided since the injection is hypertonic.2 (See Cautions: Adverse Effects.) Blood does not mix with methocarbamol injection, and blood in the syringe can either be injected with the drug or the injection of methocarbamol can be stopped when the plunger reaches the blood.2
For IM administration, no more than 500 mg (5 mL of the 100-mg/mL methocarbamol injection) should be given into each gluteal region.2 Injections may be repeated every 8 hours if necessary.2 When satisfactory relief of symptoms is achieved, patients usually can be switched to oral therapy for maintenance treatment.2
The usual initial adult oral dosage of methocarbamol is 1.5 g 4 times daily for 2-3 days.1 In patients with severe symptoms, 8 g daily in divided doses may be required initially.1 For maintenance therapy, the initial dosage can be decreased to 4-4.5 g daily in 3-6 divided doses.1
The usual adult IM or IV dose of methocarbamol is 1 g.2 Generally, oral therapy with the drug can be initiated after one injection to maintain relief of musculoskeletal pain.2 For more severe conditions or when oral administration is not feasible, additional IM or IV doses of 1 g may be administered at 8-hour intervals up to a maximum of 3 g daily for no more than 3 consecutive days.2 If necessary, the drug may be readministered IM or IV after a drug-free interval of 48 hours.2
When methocarbamol is used in the management of tetanus, the recommended initial adult dose is 1-2 g by direct IV injection.2 An additional 10-20 mL (1-2 g) may be administered by IV infusion so that a total initial dose of up to 3 g is given.2 This dosage regimen should be repeated every 6 hours until a nasogastric tube can be inserted.2 Methocarbamol tablets may then be crushed and suspended in water or saline solutions and administered through the nasogastric tube.2 Total adult oral dosage of up to 24 g daily may be required for the management of tetanus.2
For the management of tetanus in children, the recommended minimum initial IV dose of methocarbamol is 15 mg/kg or 500 mg/m2.2 This dose may be repeated every 6 hours, if necessary.2 Maintenance dosage can be given by direct IV injection or as an IV infusion.2 The total dosage should not exceed 1.8 g/m2 for 3 consecutive days.2
The most frequent adverse effects of methocarbamol are drowsiness, dizziness, and lightheadedness. Blurred vision, headache, fever, and nausea may occur after oral, IM, or IV administration of the drug. Anorexia has been reported after oral administration. Adynamic ileus occurred in one patient who received a total of 10 g of methocarbamol orally. Metallic taste, GI upset, nystagmus, diplopia, flushing, vertigo, mild muscular incoordination, syncope, hypotension, and bradycardia have occurred in patients receiving the drug IM or IV. Other adverse effects that have been reported with methocarbamol include dyspepsia, angioedema, jaundice (including cholestatic jaundice), vomiting, amnesia, confusion, insomnia, and sedation.1
Allergic reactions such as urticaria, pruritus, rash, skin eruptions, and conjunctivitis with nasal congestion may occur in patients receiving methocarbamol. Anaphylactic reactions and angioedema also have occurred.1, 2 Although most patients with methocarbamol-induced syncope recover spontaneously, epinephrine, corticosteroids, and/or antihistamines have been used to increase the rate of recovery in some of these patients.2
When methocarbamol is administered IV, thrombophlebitis, sloughing, and pain at the injection site may result from extravasation. IM injection of the drug may also cause local irritation. IV injection of methocarbamol may cause a small amount of hemolysis and increased hemoglobin and red blood cells in the urine. Leukopenia may occur rarely.
Although a causal relationship has not been established, seizures have been reported during IV administration of methocarbamol.
Precautions and Contraindications
Patients should be warned that methocarbamol may impair their ability to perform hazardous activities requiring mental alertness or physical coordination such as operating machinery or driving a motor vehicle.1, 2
Methocarbamol injection should be used with caution in patients with known or suspected seizure disorders.2
Methocarbamol injection should not be administered to patients with impaired renal function,2 The polyethylene glycol vehicle of methocarbamol injection may worsen preexisting acidosis and urea retention; although the amount present in the preparation is well within limits of safety, caution is advised.2 Methocarbamol is contraindicated in patients with known hypersensitivity to the drug or to any ingredient in the formulation.1, 2
Safety and efficacy of methocarbamol tablets have not been established in pediatric patients younger than 16 years of age.1
Safety and efficacy of methocarbamol injection have not been established in pediatric patients, except for in the management of tetanus.2
Because of the risk of injury, skeletal muscle relaxants should generally be avoided in geriatric patients.111
It is not known whether methocarbamol can cause fetal harm or affect reproductive capacity.1 Animal reproductive studies have not been performed; however, there have been reports of fetal and congenital abnormalities following in utero exposure to the drug.1 Methocarbamol should not be used during pregnancy, particularly during early pregnancy, unless the potential benefits justify the possible risks to the fetus.1
Methocarbamol is distributed into milk in dogs; however, it is not known whether the drug or its metabolites are distributed into human milk.1 Methocarbamol should be used with caution in nursing women.1
Additive CNS depression may occur when methocarbamol is administered concomitantly with other CNS depressants, including alcohol.1, 2 If methocarbamol is used concomitantly with other depressant drugs, caution should be used to avoid overdosage.1, 2
One patient with myasthenia gravis controlled by pyridostigmine experienced severe weakness after taking methocarbamol. Because methocarbamol may inhibit the effect of pyridostigmine, the drug should be used with caution in patients with myasthenia gravis receiving anticholinesterase agents.1
The urine of some patients receiving methocarbamol has been reported to turn brown, black, blue, or green on standing. Methocarbamol may produce false-positive results for urine 5-hydroxyindolacetic acid (5-HIAA) when nitrosonaphthol reagent is used (quantitative method of Udenfriend) and for urine vanillylmandelic acid (VMA) by the screening method of Gitlow (but not by the quantitative method of Sunderman), probably as a result of a metabolite of the drug.
Limited information is available on the acute toxicity of methocarbamol. Extreme drowsiness reportedly occurred in one adult who ingested 22-30 g of the drug; treatment was symptomatic and recovery was uneventful. Another adult survived an ingestion of 30-50 g; the principal symptom was drowsiness.
Management of overdosage includes symptomatic and supportive treatment. Supportive measures include maintenance of an adequate airway, monitoring urinary output and vital signs, and administration of IV fluids if necessary.1 The usefulness of hemodialysis in managing overdosage is not known.1
Methocarbamol is a CNS depressant with sedative and skeletal muscle relaxant effects. The precise mechanism of action of the drug is not known. The skeletal muscle relaxant effects of orally and parenterally administered methocarbamol are minimal and are probably related to its sedative effect. The drug does not directly relax skeletal muscle, and unlike neuromuscular blocking agents, does not depress neuronal conduction, neuromuscular transmission, or muscle excitability.
Methocarbamol is rapidly and almost completely absorbed from the GI tract.22 Blood or serum concentrations of methocarbamol required for sedative, skeletal muscle relaxant, or toxic effects are not known. Following oral administration of a single dose of methocarbamol, peak blood or serum concentrations of the drug appear to be attained in approximately 1-2 hours; the onset of action is usually within 30 minutes. Data from an unpublished study indicate that peak blood concentrations (measured as total carbamates and expressed in terms of methocarbamol) average 16.5 mcg/mL following a single 2-g oral dose, while data from a published study (using an assay relatively specific for methocarbamol) indicate that peak serum concentrations average 29.8 mcg/mL following the same dose. Data from the unpublished study also indicate that after IV administration of 1 g of methocarbamol at a rate of 300 mg/minute, blood concentrations of 19 mcg/mL are attained immediately and that the onset of action is almost immediate.
In dogs, methocarbamol is widely distributed, with highest concentrations attained in the kidney and liver; lower concentrations are attained in the lungs, brain, and spleen, and low concentrations are attained in heart and skeletal muscle.29 The drug and/or its metabolites cross the placenta in dogs.29 It is not known if methocarbamol is distributed into milk in humans.1
Methocarbamol has a serum half-life of 0.9-1.8 hours. Methocarbamol is extensively metabolized, presumably in the liver, by dealkylation and hydroxylation.1, 23 The drug is eliminated in urine principally as metabolites.1, 22, 23 Based on limited data, about 10-15% of a single oral dose is excreted in urine as unchanged drug, about 40-50% as the glucuronide and sulfate conjugates of 3-(2-hydroxyphenoxy)-1,2-propanediol-1-carbamate and 3-(4-hydroxy-2-methoxyphenoxy)-1,2-propanediol-1-carbamate, and the remainder as unidentified metabolites. Following oral administration of radiolabeled methocarbamol in healthy individuals, very small amounts of radioactivity were excreted in feces.23
Methocarbamol, a carbamate derivative of guaifenesin, is a centrally acting skeletal muscle relaxant. Methocarbamol occurs as a white powder.1, 2 The drug is sparingly soluble in water and chloroform, and is soluble in propylene glycol and soluble in alcohol only upon heating.1 The pH of methocarbamol injection is 3.5-6.2
Methocarbamol tablets should be stored in tight containers at a temperature of 20-25°C and protected from light and moisture.1, 102 Methocarbamol injection should be stored at 20-25°C, but may be exposed to temperatures of 15-30°C.2
IV infusion solutions prepared from methocarbamol injection should not be refrigerated since precipitation and haze formation may occur. At 4°C, precipitation and haze occur at methocarbamol concentrations of about 15 mg/mL or higher when the drug is diluted in 5% dextrose, 5% dextrose and 0.45% sodium chloride, or 0.9% sodium chloride injection and also occasionally occur at lower concentrations. Methocarbamol solutions containing 4 mg/mL in sterile water for injection or 5% dextrose or 0.9% sodium chloride injection are stable for 6 days at room temperature. Because formation of haze in diluted solutions of methocarbamol may be unpredictable, all diluted solutions of the drug should be inspected visually for presence of a haze prior to administration regardless of storage conditions. Because compatibility of methocarbamol injection with other drugs may depend on several factors (e.g., the concentration of the drugs, specific diluents used, resulting pH, temperature), specialized references should be consulted for specific compatibility information.
Excipients in commercially available drug preparations may have clinically important effects in some individuals; consult specific product labeling for details.
Please refer to the ASHP Drug Shortages Resource Center for information on shortages of one or more of these preparations.
Routes | Dosage Forms | Strengths | Brand Names | Manufacturer |
|---|---|---|---|---|
Oral | Tablets | 500 mg* | ||
750 mg* | Methocarbamol Tablets | |||
Tablets, film-coated | 500 mg | |||
750 mg | Robaxin®-750 | Endo | ||
Parenteral | Injection | 100 mg/mL | Robaxin® |
* available from one or more manufacturer, distributor, and/or repackager by generic (nonproprietary) name
Only references cited for selected revisions after 1984 are available electronically.
1. Endo Pharmaceuticals. Robaxin®/Robaxin®-750 (methocarbamol) tablet, film-coated prescribing information. Malvern, PA; 2019 Jan.
2. West-ward. Robaxin® (methocarbamol) injection prescribing information. Eatontown, NJ; 2017 Oct.
18. SCHWAB RS. MUSCLE RELAXANTS. Practitioner . 1964; 192:104-8. [PubMed 14106636]
22. Forist AA, Judy RW. Comparative pharmacokinetics of chlorphenesin carbamate and methocarbamol in man. J Pharm Sci . 1971; 60:1686-8. [PubMed 5133920]
23. Bruce RB, Turnbull LB, Newman JH. Metabolism of methocarbamol in the rat, dog, and human. J Pharm Sci . 1971; 60:104-6. [PubMed 5548215]
29. CAMPBELL AD, COLES FK, EUBANK LL et al. Distribution and metabolism of methocarbamol. J Pharmacol Exp Ther . 1961; 131:18-25. [PubMed 13690197]
102. West-Ward. Methocarbamol tablet prescribing information. Eatontown, NJ. 2017 Sep.
103. See S, Ginzburg R. Skeletal muscle relaxants. Pharmacotherapy . 2008; 28:207-13. [PubMed 18225966]
104. van Tulder MW, Touray T, Furlan AD et al. Muscle relaxants for non-specific low back pain. Cochrane Database Syst Rev . 2003; :CD004252. [PubMed 12804507]
105. Roelofs PD, Deyo RA, Koes BW et al. Non-steroidal anti-inflammatory drugs for low back pain. Cochrane Database Syst Rev . 2008; :CD000396. [PubMed 18253976]
106. Chou R, Qaseem A, Snow V et al. Diagnosis and treatment of low back pain: a joint clinical practice guideline from the American College of Physicians and the American Pain Society. Ann Intern Med . 2007; 147:478-91. [PubMed 17909209]
107. Institute for Clinical Systems Improvement. Health care guideline: adult acute and subacute low back pain. 15th ed. Bloomington, MN; 2012 Jan. From the ICSI website [Web]
108. Toth PP, Urtis J. Commonly used muscle relaxant therapies for acute low back pain: a review of carisoprodol, cyclobenzaprine hydrochloride, and metaxalone. Clin Ther . 2004; 26:1355-67. [PubMed 15530999]
109. Qaseem A, Wilt TJ, McLean RM et al. Noninvasive Treatments for Acute, Subacute, and Chronic Low Back Pain: A Clinical Practice Guideline From the American College of Physicians. Ann Intern Med . 2017; 166:514-530. [PubMed 28192789]
110. Friedman BW, Cisewski D, Irizarry E et al. A Randomized, Double-Blind, Placebo-Controlled Trial of Naproxen With or Without Orphenadrine or Methocarbamol for Acute Low Back Pain. Ann Emerg Med . 2018; 71:348-356.e5. [PubMed 29089169]
111. Spence MM, Shin PJ, Lee EA et al. Risk of injury associated with skeletal muscle relaxant use in older adults. Ann Pharmacother . 2013 Jul-Aug; 47:993-8. [PubMed 23821610]
112. Friedman BW, Irizarry E, Solorzano C et al. A Randomized, Placebo-Controlled Trial of Ibuprofen Plus Metaxalone, Tizanidine, or Baclofen for Acute Low Back Pain. Ann Emerg Med . 2019; [PubMed 30955985]
113. Friedman BW, Dym AA, Davitt M et al. Naproxen With Cyclobenzaprine, Oxycodone/Acetaminophen, or Placebo for Treating Acute Low Back Pain: A Randomized Clinical Trial. JAMA . 2015; 314:1572-80. [PubMed 26501533]