Veterinary Drug Handbook (VDH) is the reference veterinarians turn to when they want an independent source of information on the drugs that are used in veterinary medicine today.


METHOCARBAMOL

Chemistry - A centrally acting muscle relaxant related structurally to guaifenesin, methocarbamoloccurs as a fine, white powder with a characteristic odor. In water, it has a solubility 25 mg/ml. ThepH of commercial injection is approximately 4-5.

Storage, Stability, Compatibility

Methocarbamol tablets should be stored at room temperaturein tight containers; the injection should be stored at room temperature and not frozen. Solutionsprepared for IV infusion should not be refrigerated as a precipitate may form. Because a haze orprecipitate may form, all diluted intravenous solutions should be physically inspected before administration.

Pharmacology - METHOCARBAMOL

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 relaxcontracted skeletal muscles. The drug has a secondary sedative effect.
Uses, Indications - In dogs and cats, methocarbamol is indicated (FDA approved) "as adjunctivetherapy of acute inflammatory and traumatic conditions of the skeletal muscle and to reducemuscular spasms." In horses, intravenous use is indicated (FDA approved) "as adjunctive therapyof acute inflammatory and traumatic conditions of the skeletal muscle to reduce muscular spasms, and effect striated muscle relaxation." (Package insert; Robaxin® -V - Robins)

Pharmacokinetics - METHOCARBAMOL

Limited pharmacokinetic data is available in veterinary species. In humans, methocarbamol ha an onset of action of about 30 minutes after oral administration. Peak levelsoccur 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-lifeof methocarbamol in the horse is approximately 60-70 minutes. Guaifenesin is a minor metaboliteof methocarbamol, but because of very low concentrations, it probably has no clinical effect in thehorse.
Contraindications/Precautions - Because the injectable product contains polyethylene glycol300, the manufacturer lists known or suspected renal pathology as a contraindication to injectablemethocarbamol therapy. Polyethylene glycol 300 has been noted to increase preexisting acidosisand urea retention in humans with renal impairment.
Methocarbamol should be used with caution during pregnancy as studies demonstrating its safetyduring pregnancy are lacking. Methocarbamol should not be used in patients hypersensitive to it orin animals to be used for food purposes.
Do not administer subcutaneously and avoid extravasation. Do not exceed 2 ml per minute wheninjecting 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 rightingreflex, 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 indicatedif the overdose was recent. Do not induce emesis if the patient's continued consciousness is notassured. Other symptoms should be treated if severe and in a supportive manner.

Drug Interactions

Because methocarbamol is a CNS depressant, additive depression may occurwhen given with other CNS depressant agents.
One patient (human) with myasthenia gravis and taking pyridostigmine, developed severeweakness after receiving methocarbamol.
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