Methocarbamol, sold under the trade name Robaxin, is a muscle relaxant commonly used in horses to treat muscle pain secondary to spasticity, tetanus, or tying-up.Cunningham, F. E., et al. "The pharmacokinetics of methocarbamol in the thoroughbred race horse." Journal of veterinary pharmacology and therapeutics 15.1 (1992): 96-100. Both oral and injectable forms are on the market. Its use is regulated by many equine competitive governing bodies, since it is thought to have a CNS depressant effect and produces the metabolite guaifenesin, which is a pre-anesthetic veterinary drug.Muir, W.W., Sams, R.A. & Ashcraft, S. (1984) Pharmacologic and pharmacokinetic properties of methocarbamol in the horse. American Journal of Veterinary Research, 45, 2256–2260. Guaifenesin has been found in the plasma of horses receiving oral, but not intravenous, methocarbamol, likely due to the high first-pass effect of the drug.Rumpler, M. J., Colahan, P., Sams, R. A. The pharmacokinetics of methocarbamol and guaifenesin after single intravenous and multiple-dose oral administration of methocarbamol in the horse. J. vet. Pharmacol. Therap. 37, 25–34
