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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.
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.
Unlike other carbamates such as meprobamate and its prodrug carisoprodol, methocarbamol has greatly reduced abuse potential. Studies comparing it to lorazepam (Ativan) and diphenhydramine (Benadryl), 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.
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, 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 & 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.
Because of potential for side-effects, this drug is considered to be a high-risk medication for the elderly.
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- See NCQA’s HEDIS Measure: Use of High Risk Medications in the Elderly
- R.S. Murphey, U.S. Patent 2,770,649 (1956)
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