Chemistry - Formerly known as glyceryl guaiacolate , guaifenesin occurs as a white to slightly gray, crystalline powder which may have a characteristic odor. It is nonhygroscopic and melts between 78° - 82°C. One gram is soluble in 15 ml of water and it is soluble in alcohol, propylene glycol 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 melting point). It should be stored in well closed containers.
When dissolved into aqueous solutions, guaifenesin may slightly precipitate out of solution when the temperature is less than 22°C (72°F). Slight warming and agitation generally resolubilizes the drug. A microwave oven has been suggested for heating and dissolving the drug. It is recommended that the solution be prepared freshly before use, but a 10% solution (in sterile water) may apparently be stored safely at room temperature for up to one week with only slight precipitation occurring.
Guaifenesin is physically compatible with sterile water or D5W. It is also reportedly compatible with ketamine, pentobarbital, thiamylal, thiopental, and xylazine.
Pharmacology - While the exact mechanism of action for the muscle relaxant effects are not known, it is believed that guaifenesin acts centrally by depressing or blocking nerve impulse transmission at the internuncial neuron level of the subcortical areas of the brain, brainstem and spinal 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 include transient mild decreases in blood pressure and increases in cardiac rate. Gastrointestinal motility may 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 and restraint as an adjunct to anesthesia for short procedures (30-60 minutes) in large and small animal species.
In human medicine, guaifenesin has long been touted as an oral expectorant, but definitive proof of its efficacy is lacking.
Pharmacokinetics - The pharmacokinetics of guaifenesin have not been thoroughly studied in most species. When administered alone to horses IV, recumbency usually occurs within 2 minutes and light (not surgical level) restraint persists for about 6 minutes. Muscle relaxation reportedly persists for 10-20 minutes after a single dose.
Guaifenesin is conjugated in the liver and excreted into the urine. A gender difference in the elimination half-life of guaifenesin in ponies has been demonstrated, with males having a t1/2 of approximately 85 minutes, and females a t1/2 of about 60 minutes. Guaifenesin reportedly crosses the placenta, but adverse effects in newborns of mothers who received guaifenesin have not been described.
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 mild decrease in blood pressure and increase in cardiac rate can be seen. Thrombophlebitis has been reported after IV injection, and perivascular administration may cause some tissue reaction. Hemolysis may occur in solutions containing greater than a 5% concentration of guaifenesin, but some sources state this is insignificant at even a 15% concentration.
Overdosage - The margin of safety is reportedly 3 times the usual dose. Symptoms of apneustic breathing, nystagmus, hypotension, and contradictory muscle rigidity are associated with toxic levels of the drug. No specific antidote is available. It is suggested that treatment be supportive until the 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 not elucidate on the actual interaction. It may be logical to assume that other anticholinesterase agents (neostigmine, pyridostigmine, edrophonium) may also be contraindicated.
a) 110 mg/kg IV, give first 1/3-1/2 of dose until horse falls gently, then give remainder unless respiratory or cardiovascular effects are observed. (Package Insert, Guailaxin® - Robins)
b) Guaifenesin only: 66 - 132 mg/kg IV; or guaifenesin 44 - 88 mg/kg IV with 2.2 - 6.6 mg/kg thiamylal (Muir )
c) 55 - 110 mg/kg IV (Mandsager 1988)
d) For anesthesia: 100 mg/kg IV combined with barbiturate in 5% dextrose. As an expectorant: 3 mg/kg PO (Robinson 1987)
Monitoring Parameters -
1) Level of muscle relaxation
2) Cardiac and respiratory rate
Dosage Forms/Preparations/FDA Approval Status/Withholding Times -
Guaifenesin Sterile Powder for Injection 50 gram for reconstitution in 4 oz and 32 oz containers; Guailaxin® (Fort Dodge), Gecolate® (Summit Hill); (Rx) Approved for use in horses.
Guaifenesin Injection 50mg/ml 1000ml; Gecolate® (Summit Hill); generic (Phoenix); (Rx) Approved for use in horses.
Human-Approved Products: No parenteral preparations are approved. There are many OTC oral expectorant/cough preparations on the market.
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