Chemistry - A centrally acting muscle relaxant related structurally to guaifenesin, methocarbamol occurs as a fine, white powder with a characteristic odor. In water, it has a solubility 25 mg/ml. The pH of commercial injection is approximately 4-5.
Storage/Stability/Compatibility - Methocarbamol tablets should be stored at room temperature in tight containers; the injection should be stored at room temperature and not frozen. Solutions prepared for IV infusion should not be refrigerated as a precipitate may form. Because a haze or precipitate may form, all diluted intravenous solutions should be physically inspected before administration.
Pharmacology - Methocarbamol’s exact mechanism of causing skeletal muscle relaxation is unknown. It is thought to work centrally, perhaps by general depressant effects. It has no direct relaxant effects on striated muscle, nerve fibers, or the motor endplate. It will not directly relax contracted skeletal muscles. The drug has a secondary sedative effect.
Uses/Indications - In dogs and cats, methocarbamol is indicated (FDA approved) “as adjunctive therapy of acute inflammatory and traumatic conditions of the skeletal muscle and to reduce muscular spasms.” In horses, intravenous use is indicated (FDA approved) “as adjunctive therapy of acute inflammatory and traumatic conditions of the skeletal muscle to reduce muscular spasms, and effect striated muscle relaxation.” (Package insert; Robaxin® -V - Robins)
Pharmacokinetics - Limited pharmacokinetic data is available in veterinary species. In humans, methocarbamol ha an onset of action of about 30 minutes after oral administration. Peak levels occur approximately 2 hours after dosing. Serum half-life is about 1-2 hours. The drug is metabolized and the inactive metabolites are excreted into the urine and the feces (small amounts).
In horses, plasma clearances appear to be dose dependent after IV administration (Muir, Sams, and Ashcraft 1984), lower clearances were measured after higher doses were given. The serum half-life of methocarbamol in the horse is approximately 60-70 minutes. Guaifenesin is a minor metabolite of methocarbamol, but because of very low concentrations, it probably has no clinical effect in the horse.
Contraindications/Precautions - Because the injectable product contains polyethylene glycol 300, the manufacturer lists known or suspected renal pathology as a contraindication to injectable methocarbamol therapy. Polyethylene glycol 300 has been noted to increase preexisting acidosis and urea retention in humans with renal impairment.
Methocarbamol should be used with caution during pregnancy as studies demonstrating its safety during pregnancy are lacking. Methocarbamol should not be used in patients hypersensitive to it or in animals to be used for food purposes.
Do not administer subcutaneously and avoid extravasation. Do not exceed 2 ml per minute when injecting IV in dogs and cats.
Adverse Effects/Warnings - Side effects can include sedation, salivation, emesis, lethargy, weakness and ataxia in dogs and cats. Sedation and ataxia are possible in horses. Because of its CNS depressant effects, methocarbamol may impair the abilities of working animals.
Overdosage - Overdosage is generally characterized by CNS depressant effects (loss of righting reflex, prostration). Excessive doses in dogs and cats may be represented by emesis, salivation, weakness and ataxia. If the overdose is after oral administration, emptying the gut may be indicated if the overdose was recent. Do not induce emesis if the patient’s continued consciousness is not assured. Other symptoms should be treated if severe and in a supportive manner.
Drug Interactions - Because methocarbamol is a CNS depressant, additive depression may occur when given with other CNS depressant agents.
One patient (human) with myasthenia gravis and taking pyridostigmine, developed severe weakness after receiving methocarbamol.
a) For moderate conditions: 4.4 - 22 mg/kg IV to effect; for severe conditions: 22 - 55 mg/kg IV (Package insert, Robaxin®-V — Robins)
b) 15 - 25 mg/kg IV by slow infusion (Robinson 1987)
c) To give orally: Use 2-3 times the recommended IV dose. (Cunningham, Fisher et al. 1992)
Monitoring Parameters -
1) Level of muscle relaxation/sedation
Client Information - Animal’s urine color may darken, but need not be of concern.
Dosage Forms/Preparations/FDA Approval Status/Withholding Times -
Methocarbamol Tablets 500 mg; Robaxin®-V (Fort Dodge); (Rx) Approved for use in dogs and cats.
Methocarbamol Injection 100 mg/ml in vials of 20 ml and 100 ml; Robaxin®-V (Fort Dodge); (Rx) Approved for use in dogs, cats, and horses not intended for food.
Methocarbamol Tablets 500 mg, 750 mg; Robaxin® (Robins); Robaxin-750® (Robins); Generic (Rx)
Methocarbamol Injection 100 mg/ml in 10 ml vials; Robaxin® ;(Robins); generic (Rx)
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