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.


Chemistry - Formerly known as glyceryl guaiacolate, guaifenesin occurs as a white to slightlygray, crystalline powder which may have a characteristic odor. It is nonhygroscopic and meltsbetween 78° - 82°C. One gram is soluble in 15 ml of water and it is soluble in alcohol, propyleneglycol and glycerin. Guaifenesin may also be known as glyceryl guaiacolate, GG, or guaiphenesin.

Storage, Stability, Compatibility

Guaifenesin is stable in light and heat (less than meltingpoint). It should be stored in well closed containers.
When dissolved into aqueous solutions, guaifenesin may slightly precipitate out of solution whenthe temperature is less than 22°C (72°F). Slight warming and agitation generally resolubilizes thedrug. A microwave oven has been suggested for heating and dissolving the drug. It is recommendedthat the solution be prepared freshly before use, but a 10% solution (in sterile water) mayapparently be stored safely at room temperature for up to one week with only slight precipitationoccurring.
Guaifenesin is physically compatible with sterile water or D5W. It is also reportedly compatiblewith ketamine, pentobarbital, thiamylal, thiopental, and xylazine.

Pharmacology - GUAIFENESIN

While the exact mechanism of action for the muscle relaxant effects are notknown, it is believed that guaifenesin acts centrally by depressing or blocking nerve impulsetransmission at the internuncial neuron level of the subcortical areas of the brain, brainstem andspinal cord. It relaxes both the laryngeal and pharyngeal muscles, thus allowing easier intubation.
Guaifenesin also has mild intrinsic analgesic and sedative qualities.
Guaifenesin causes an excitement-free induction and recovery from anesthesia in horses. It produces relaxation of skeletal muscles, but does not affect diaphragmatic function and has little, if any, effects on respiratory function at usual doses. Possible effects on the cardiovascular system includetransient mild decreases in blood pressure and increases in cardiac rate. Gastrointestinal motilitymay be increased, but generally no adversity is seen with this.
Guaifenesin potentiates the activity of preanesthetic and anesthetic agents.

Uses, Indications

In veterinary medicine, guaifenesin is used to induce muscle relaxation andrestraint as an adjunct to anesthesia for short procedures (30-60 minutes) in large and small animalspecies.
In human medicine, guaifenesin has long been touted as an oral expectorant, but definitive proof ofits efficacy is lacking.

Pharmacokinetics - GUAIFENESIN

The pharmacokinetics of guaifenesin have not been thoroughly studied inmost species. When administered alone to horses IV, recumbency usually occurs within 2 minutesand light (not surgical level) restraint persists for about 6 minutes. Muscle relaxation reportedlypersists for 10-20 minutes after a single dose.
Guaifenesin is conjugated in the liver and excreted into the urine. A gender difference in theelimination half-life of guaifenesin in ponies has been demonstrated, with males having a t1/2 ofapproximately 85 minutes, and females a t1/2 of about 60 minutes. Guaifenesin reportedly crossesthe placenta, but adverse effects in newborns of mothers who received guaifenesin have not beendescribed.
Contraindications/Precautions - The manufacturer states that the use of physostigmine is contraindicated with guaifenesin (see Drug Interactions).

Adverse Effects, Warnings

At usual doses, side effects are transient and generally minor. A milddecrease in blood pressure and increase in cardiac rate can be seen. Thrombophlebitis has beenreported after IV injection, and perivascular administration may cause some tissue reaction.
Hemolysis may occur in solutions containing greater than a 5% concentration of guaifenesin, butsome sources state this is insignificant at even a 15% concentration.
Overdosage - The margin of safety is reportedly 3 times the usual dose. Symptoms of apneusticbreathing, nystagmus, hypotension, and contradictory muscle rigidity are associated with toxiclevels of the drug. No specific antidote is available. It is suggested that treatment be supportive untilthe drug is cleared to sub-toxic levels.

Drug Interactions

Drug interactions with guaifenesin are not well studied. The manufacturer(Robins) states that physostigmine is contraindicated in horses receiving guaifenesin, but does notelucidate on the actual interaction. It may be logical to assume that other anticholinesterase agents(neostigmine, pyridostigmine, edrophonium) may also be contraindicated.

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