Source: Medicines & Healthcare Products Regulatory Agency (GB) Publisher: neuraxpharm Arzneimittel GmbH, Elisabeth-Selbert-Str.23, 40764, Langenfeld, Germany
Pharmacotherapeutic group: Muscle relaxants, centrally acting agents; carbamic acid esters
ATC Code: M03BA03
Methocarbamol is a centrally acting muscle relaxant. Its muscle relaxing action is the result of the inhibition of polysynaptic reflexes in the spinal marrow and subcortical centres. Methocarbamol, at the therapeutic dose, does not affect the physiological tonus and contractility of the skeletal muscles as well as the motility of non-striated muscles, and has no action on the motor end plate.
After oral administration methocarbamol is absorbed quickly and completely. The active substance can be detected in blood already 10 minutes after intake and produces peak blood concentrations after 30-60 minutes.
Plasma half-life of methocarbamol is ca. 2 hours. Methocarbamol and its two metabolites are bound to glucuronic and sulfuric acid and are eliminated nearly exclusively via the kidneys. About half the dose administered is excreted into urine within 4 hours, only a small portion of which is eliminated as unchanged methocarbamol.
The clearance of methocarbamol in renally-impaired patients on maintenance haemodialysis was reduced about 40% compared to a normal population, although the mean elimination half-life in these two groups was similar (1.2 versus 1.1 hours, respectively).
In patients with cirrhosis secondary to alcohol abuse, the mean total clearance of methocarbamol was reduced approximately 70% compared to a normal population (11.9 L/hr), and the mean elimination half-life was extended to approximately 3.4 hours. The fraction of methocarbamol bound to plasma proteins was decreased to approximately 40 to 45% compared to 46 to 50% in an age and weight-matched normal population.
The acute toxicity of methocarbamol is comparatively low. Signs of intoxication in animal studies are ataxia, catalepsy, convulsions and coma.
Studies on chronic toxicity have not been performed.
Studies to determine a potential toxicity on reproduction have not been carried out.
In vitro and in vivo studies on genetic toxicity of methocarbamol did not reveal evidence of a mutagenic potential.
Long-term studies for evaluation of a carcinogenic potential have not been performed.
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