Methocarbamol is a muscle relaxant used to treat skeletal muscle spasms, with some effect on the central nervous system. One of its actions is to block nerve impulses (such as pain sensations) to the brain. Methocarbamol is locally available as a human tablet preparation of commercial name “Robaxin”. It has therapeutic application in the horse and is used for the treatment of acute inflammatory and traumatic conditions of the skeletal muscle to reduce muscle spasm and effect muscle relaxation. Within the NHA Classification it is “Class 4 - substances which have a generally accepted veterinary (therapeutic) use in the racehorse but which have the potential to affect performance.”
Methocarbamol is a substance with an Asian Racing Federation screening limit and as member of this federation, the NHA has adopted this limit. This screening limit of 100 ng/ml in urine applies to: “Methocarbamol (restricted to single oral or IV treatment of no more than 5 grams)”
Within this guidance above it is clearly stated that it applies only to a single administration. The reason for this is that multiple dose of this substance have been reported to result in an accumulation of methocarbamol. This is non-predictable between different horses. This accumulation implies that multiple doses have an excretion time which is much longer than that of a single dose.
In oral studies there have been evidence of recycling during some trials. Recycling is when the preparation remains present in stable areas after administration (for example spilling, dripping or spitting by the horse), or when the substance is excreted in the urine and faeces and re-enters the horse. This can be reduced if the stable is frequently and thoroughly cleaned and with the realisation that it is not highly soluble in cold water. Recycling can be non-predictable between horses as well as in different stable environments.
It is recommended that Methocarbamol only be used as single dose treatment to the racehorse, with a detection time of about 48 hours (two days). With a detection of two days a withdrawal time period significantly longer than this must apply. In isolation of this statement and guidance is the fact that a withdrawal period of several weeks must be applied when multiple doses are administered as part of a treatment regimen.
Consult your veterinarian with regards to suitable withdrawal periods. The NHA does not assume any legal, professional or other responsibility or duty whatsoever as to the accuracy of the data presented in this document. Please note that reliance on and use of the above information does not absolve or diminish a trainer or owner from being responsible for ensuring that the horse complies with the rules relating to the presence of prohibited substances when presenting a horse for a race.
