Guaifenesin
**Guaifenesin** (also known as glyceryl guaiacolate or GG) is a dual-purpose pharmacological agent. In veterinary medicine, it is primarily utilized as a **parenteral muscle relaxant** to facilitate excitement-free induction and recovery during short anesthetic procedures (30-60 minutes), particularly in large animals. * **Anesthetic Adjunct**: Provides skeletal muscle relaxation without compromising diaphragmatic function. * **Oral Expectorant**: Used in both human and veterinary medicine to manage respiratory conditions by thinning mucus, though definitive proof of clinical efficacy remains debated. **Clinical Pearl**: Guaifenesin is not an anesthetic on its own; it potentiates the activity of preanesthetic and anesthetic agents and must be used in balanced protocols (e.g., "Triple Drip" or GKX). It possesses mild intrinsic analgesic and sedative qualities.
Mechanism: * **Muscle Relaxation**: 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 โ selective relaxation of skeletal muscles (including laryngeal and pharyngeal muscles, easing intubation) while sparing respiratory muscles. * **Expectorant Action**: Believed to stimulate gastric vagal receptors โ triggers efferent parasympathetic reflexes โ increases respiratory tract fluid volume and decreases mucus viscosity โ facilitates removal of secretions via ciliary action and coughing.
Dosing by species
- Using 'GKX' for induction/maintenance ยท Induce with 2.2 mL/kg IV. Maintain with approximately 2.2 mL/kg/hr IV ยท IV ยท CRI ยท Mix 1000 mL of 5% (50 mg/mL) guaifenesin with 1000 mg of ketamine ยฑ 0-100 mg of xylazine.
- Using 'GKX' ยท Administer at a rate of 2-4 mL/kg ยท IV ยท Once then slowed ยท Given rapidly until animal is recumbent and then slowed to maintain desired anesthesia plane.
- Using 'GKX' for induction/maintenance ยท Induce with 2.2 mL/kg IV. Maintain with approximately 2.2 mL/kg/hr IV ยท IV ยท CRI ยท Mix 1000 mL of 5% (50 mg/mL) guaifenesin with 1000 mg of ketamine ยฑ 0-100 mg of xylazine.
- Using 'GKX' ยท Administer at a rate of 2-4 mL/kg ยท IV ยท Once then slowed ยท Given rapidly until animal is recumbent and then slowed to maintain desired anesthesia plane.
- General ยท 55-110 mg/kg ยท IV ยท Once
- General anesthesia adjunct ยท 110 mg/kg ยท IV ยท Once ยท Give first 1/3-1/2 of dose until horse falls gently, then give remainder unless respiratory or cardiovascular effects are observed.
- Muscle relaxation in balanced anesthesia ยท A CRI of ketamine at 0.8-2.3 mg/kg/hr with guaifenesin at 18-60 mg/kg/hr ยท IV ยท CRI ยท May reduce MAC requirements by 50%.
Routes of administration
Contraindications
- Concurrent use with physostigmine
- Concentrations greater than 5% in ruminants (due to hemolysis risk)
Adverse effects
- Mild hypotensive effect
- Increase in cardiac rate
- Thrombophlebitis (IV administration)
- Perivascular tissue reaction
- Hemolysis (especially in ruminants with >5% solutions)
- Increased gastrointestinal motility
Drug interactions
- Physostigmine ยท Contraindicated in horses receiving guaifenesin; exact interaction mechanism not elucidated by manufacturer.
- Anticholinesterase agents (neostigmine, pyridostigmine, edrophonium) ยท Theoretically contraindicated based on the physostigmine interaction.
Monitoring
- Level of muscle relaxation
- Cardiac rate
- Respiratory rate
Overdose
* **Margin of Safety**: Reportedly 3 times the usual dose. * **Clinical Signs**: Apneustic breathing, nystagmus, hypotension, and contradictory muscle rigidity are associated with toxic levels. * **Toxicity Reports**: In dogs receiving 415-830 mg/kg, signs included hypothermia, mild tremors, ataxia, and vomiting. In cats receiving 132 mg/kg, signs included lethargy and anorexia. * **Treatment**: No specific antidote is available. Treatment is entirely **supportive** until the drug is cleared to sub-toxic levels.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.