Also Known As: Methocarbamol, Robaxin
Methocarbamol (trade name Robaxin, which is marketed by Actient Pharmaceuticals LLC in the United States and Pfizer in Canada) is a central muscle relaxant used to treat skeletal muscle spasms. It is the carbamate of guaifenesin, but does not produce guaifenesin as a metabolite, since 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. Unlike other carbamates such as carisoprodol and meprobamate, 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, upset stomach, flushing, blurred vision, and fever. Serious side-effects include the development of a severe skin rash or itching, slow heart rate, fainting, jaundice, persistent nausea/vomiting, stomach/abdominal pain, mental/mood changes, clumsiness, trouble urinating, signs of infection.  In addition, methocarbamol may cause urine to turn black, blue, or green. However, this effect is harmless.
Because of potential for side-effects, this drug is on the list for High-Risk Medications in the elderly. (See NCQA’s HEDIS Measure: Use of High Risk Medications in the Elderly, http://www.ncqa.org/Portals/0/Newsroom/SOHC/Drugs_Avoided_Elderly.pdf).