|ATC code||M03BA03 (WHO)|
|Biological half-life||1.14–1.24 hours|
|Chemical and physical data|
|Molar mass||241.241 g/mol|
|3D model (Jmol)||Interactive image|
|(what is this?)|
Methocarbamol is a central muscle relaxant used to treat skeletal muscle spasms. Under the trade name Robaxin, it is marketed by Actient Pharmaceuticals in the United States and Pfizer in Canada. The mechanism of action of methocarbamol is currently unknown, but may involve the inhibition of carbonic anhydrase. The muscle relaxant effects of methocarbamol are largely attributed to central depressant effects; however, peripheral effects of methocarbamol to prolong muscle refractory period have also been reported. As of 2015 the cost for a typical month of medication in the United States is less than 25 USD.
Potential side-effects include: drowsiness, dizziness, clumsiness (ataxia), upset stomach, flushing, blurred vision, and fever. Both tachycardia (fast heart rate) and bradycardia (slow heart rate) have been reported; these can be serious. Other serious side-effects include the development of a severe skin rash or itching, fainting, jaundice, persistent nausea/vomiting, stomach/abdominal pain, mental/mood changes, trouble urinating, and signs of infection. If taken in large amounts at once or more than directed or as prescribed, dysphoria or suicidal thoughts may occur. In addition, methocarbamol may cause urine to turn black, blue, or green. However, this effect is harmless.
Methocarbamol has a high therapeutic index, i.e., a wide range of safe and effective dosages. Consumer (OTC) doses are in the range 3–6 g per day, while clinical doses can be as high as 24 g per day for severe conditions such as tetanus.
Unlike other carbamates such as meprobamate and its prodrug carisoprodol, methocarbamol has greatly reduced abuse potential. Studies comparing it to the benzodiazepine lorazepam and the antihistamine diphenhydramine, along with placebo, find that methocarbamol produces increased "liking" responses and some sedative-like effects, however, at higher doses dysphoria is reported. It is considered to have an abuse profile similar to, but weaker than, lorazepam.
Methocarbamol is the carbamate of guaifenesin, but does not produce guaifenesin as a metabolite, because the carbamate bond is not hydrolyzed metabolically; metabolism is by Phase I ring hydroxylation and O-demethylation, followed by Phase II conjugation. All the major metabolites are unhydrolyzed carbamates.
Methocarbamol is marketed under different names when presented in combination with other active ingredients. In combination with acetaminophen (Paracetamol), under trade names Robaxacet and Tylenol Body Pain Night, whereas Robax Platinum is the trade name for a formulation of methocarbamol and ibuprofen. A combination of methocarbamol and aspirin is marketed as Robaxisal, however in Spain the tradename Robaxisal is used for the Paracetamol combination instead of Robaxacet.
- Sica DA, Comstock TJ, Davis J, Manning L, Powell R, Melikian A, Wright G (1990). "Pharmacokinetics and protein binding of methocarbamol in renal insufficiency and normals". European Journal of Clinical Pharmacology. 39 (2): 193–4. doi:10.1007/BF00280060. PMID 2253675.
- Parr JS, Khalifah RG (1992). "Inhibition of carbonic anhydrases I and II by N-unsubstituted carbamate esters". J Biol Chem. 267 (35): 25044–25050. PMID 1460006.
- Truitt EB Jr.; Little JM. (1958). "A pharmacologic comparison of methocarbamol (AHR-85), the monocarbamate of 3-(o-methoxyphenoxy)-1,2-propanediol with chemically related interneuronal depressant drugs". J Pharmacol Exp Ther. 122 (2): 239–246. PMID 13514612.
- Crankshaw DP, Raper C (1968). "Some studies on peripheral actions of mephenesin, methocarbamol and diazepam". Br J Pharmacol. 34 (3): 579–590. doi:10.1111/j.1476-5381.1968.tb08486.x. PMID 5726787.
- Hamilton, Richart (2015). Tarascon Pocket Pharmacopoeia 2015 Deluxe Lab-Coat Edition. Jones & Bartlett Learning. p. 2. ISBN 9781284057560.
- METHOCARBAMOL – ORAL (Robaxin) side effects, medical uses, and drug interactions. Medicinenet.com. Retrieved on 2011-11-09.
- Methocarbamol: MedlinePlus Drug Information. Nlm.nih.gov. Retrieved on 2011-11-09.
- See NCQA’s HEDIS Measure: Use of High Risk Medications in the Elderly
- "Subjective and behavioral effects of diphenhydramine, lorazepam and methocarbamol: evaluation of abuse liability". Journal of Pharmacology and Experimental Therapeutics. Retrieved 2011-05-06.
- Methocarbamol. In: DRUGDEX System [intranet database]. Greenwood Village, Colorado: Thomson Healthcare; c1974–2009 [cited 2009 Feb 10].
- Bruce RB, Turnbull LB, Newman JH (Jan 1971). "Metabolism of methocarbamol in the rat, dog, and human". J Pharm Sci. 60 (1): 104–106. doi:10.1002/jps.2600600120. PMID 5548215.
- "New Drugs and Indications Reviewed at the May 2003 DEC Meeting" (PDF). ESI Canada. Retrieved 2008-11-14.
- "Tylenol Body Pain Night Overview and Dosage" (website). Tylenol Canada. Retrieved 2012-04-23.