May cause urine to turn black, blue or green.While the product label states that methocarbamol can cause jaundice, there is minimal evidence to suggest that methocarbamol causes liver damage. During clinical trials of methocarbamol, there were no laboratory measurements of liver damage indicators, such as serum aminotransferase (AST/ALT) levels, to confirm hepatotoxicity. Although unlikely, it is impossible to rule out that methocarbamol may cause mild liver injury with use. Elderly
Skeletal muscle relaxants are associated with an increased risk of injury among older adults. Methocarbamol appeared to be less sedating than other muscle relaxants, most notably cyclobenzaprine, but had similar increased risk of injury. Methocarbamol is cited along with "most muscle relaxants" in the 2012 Beers Criteria as being "poorly tolerated by older adults, because of anticholinergic adverse effects, sedation, increased risk of fractures," noting that "effectiveness dosages tolerated by older adults is questionable." Pregnancy
Methocarbamol is labeled by the FDA as a pregnancy category C medication. The teratogenic effects of the medication are not known and should be given to pregnant women only when clearly indicated. Overdose
There is limited information available on the acute toxicity of methocarbamol. Overdose is used frequently in conjunction with CNS depressants such as alcohol or benzodiazepines and will have symptoms of nausea, drowsiness, blurred vision, hypotension, seizures, and coma. There are reported deaths with an overdose of methocarbamol alone or in the presence of other CNS depressants. Abuse
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. Interactions
Methocarbamol may inhibit the effects of pyridostigmine bromide. Therefore, methocarbamol should be used with caution in those with myasthenia gravis taking anticholinesterase medications.Methocarbamol may disrupt certain screening tests as it can cause color interference in laboratory tests for 5-hydroxy-indoleacetic acid (5-HIAA) and in urinary testing for vanillylmandelic acid (VMA) using the Gitlow method. Pharmacology
Mechanism of action
The mechanism of action of methocarbamol has not currently been established. Its effect is thought to be localized to the central nervous system rather than a direct effect on skeletal muscles. It has no effect on the motor end plate or the peripheral nerve fiber. The efficacy of the medication is likely related to its sedative effect. Alternatively, methocarbamol may act via inhibition of acetylcholinesterase, similarly to carbamate. Pharmacokinetics
In healthy individuals, the plasma clearance of methocarbamol ranges between 0.20 and 0.80 L/h/kg. The mean plasma elimination half-life ranges between 1 and 2 hours, and the plasma protein binding ranges between 46% and 50%. The elimination half-life was longer in the elderly, those with kidney problems, and those with liver problems.