Xylazine
Xylazine is a potent **alpha-2 adrenergic agonist** widely used in veterinary medicine for its sedative, analgesic, and muscle relaxant properties. It is also frequently utilized as a reliable **emetic** (to induce vomiting) in cats. > **Clinical Pearl:** There is profound species variability in sensitivity to xylazine. **Ruminants are exquisitely sensitive**, requiring approximately 1/10th the dose used in horses to achieve the same effect. Conversely, swine are highly resistant and require 20-30 times the ruminant dose. While it provides excellent visceral analgesia (especially in horses), its use in small animals is often reserved for young, healthy patients due to its significant cardiovascular depressant effects, including bradycardia, arrhythmias, and decreased cardiac output. It is not FDA-approved for use in food-producing animals, though it is sometimes used extra-label with strict withdrawal times.
Mechanism: Xylazine stimulates presynaptic and postsynaptic **alpha-2 adrenergic receptors** in the central and peripheral nervous systems. * **Sedation & Analgesia:** Activation of central alpha-2 receptors โ decreased release of **norepinephrine** and dopamine โ profound CNS depression and analgesia. * **Muscle Relaxation:** Centrally mediated inhibition of intraneuronal transmission of impulses in the CNS. * **Cardiovascular Effects (Biphasic):** Initial stimulation of peripheral alpha-1 and alpha-2 receptors โ transient vasoconstriction and **hypertension** โ subsequent central sympatholytic effect โ **hypotension**, **bradycardia**, and potentially second-degree AV block. Cardiac output can decrease by up to 30%. * **Emesis:** Stimulates the chemoreceptor trigger zone (CRTZ), particularly in cats. * **Endocrine/Renal:** Decreases insulin release โ transient **hyperglycemia**. Decreases ADH production โ **polyuria** (especially in ruminants).
Dosing by species
- Label dosage (Note: Many believe this is too high) ยท 1.1 mg/kg IV, 1.1-2.2 mg/kg IM or SC ยท IV, IM, SC ยท Once
- Preanesthetic ยท 0.2-1 mg/kg IV, IM or SC ยท IV, IM, SC ยท Once
- Emetic ยท 0.44 mg/kg IM ยท IM ยท Once
- Analgesic ยท 0.1-1 mg/kg IV, IM or SC ยท IV, IM, SC ยท Once
- Post-operative anxiety ยท 0.1-0.5 mg/kg IV, IM or SC ยท IV, IM, SC ยท Once
- Emesis ยท 0.6 mg/kg (IM) or 1 mg/kg (SC) ยท IM/SC ยท once ยท Effective in >75% of cats. Emesis generally occurs rapidly within 10 min. Further doses depress the vomiting centre and may not result in further vomiting.
- Growth hormone suppression test ยท 100 ฮผg/kg ยท IV ยท once ยท Consult specialist texts for protocol.
- Sedative/analgesic ยท 0.5-2 mg/kg IM or SC ยท IM, SC ยท Once ยท Usually combined with atropine (0.05 mg/kg) or glycopyrrolate (0.01 mg/kg IM).
- Sedative/analgesic combination ยท Xylazine (2 mg/kg) plus butorphanol (0.2 mg/kg) IM ยท IM ยท Once
- Anesthesia combination ยท Telazol (1.5 mg/kg) plus xylazine (1.5 mg/kg) IM ยท IM ยท Once ยท May reverse xylazine with yohimbine (0.05 mg/kg IM). Can also add butorphanol 0.2 mg/kg.
Routes of administration
Contraindications
- Animals receiving epinephrine
- Active ventricular arrhythmias
- Preexisting cardiac dysfunction
- Hypotension or shock
- Respiratory dysfunction
- Severe hepatic or renal insufficiency
- Preexisting seizure disorders
- Severely debilitated animals
- Last trimester of pregnancy (particularly in cattle)
- Dehydrated or debilitated ruminants
- Urinary tract obstruction (ruminants)
- Intra-arterial injection (may cause severe seizures and collapse)
- Cardiovascular disease
- Systemic disease affecting cardiovascular performance
- Geriatric patients
- Pregnant animals
Adverse effects
- Emesis (especially in cats, occasionally dogs)
- Muscle tremors
- Bradycardia with partial A-V block
- Reduced respiratory rate and tidal volume
- Sudden movement or kicking in response to sharp auditory stimuli (especially horses)
- Increased urination (cats, cattle)
- Bloat from aerophagia (dogs, cattle)
- Sweating (horses)
- Salivation, ruminal atony, regurgitation, and diarrhea (cattle)
- Premature parturition (cattle)
- Transient hypertension followed by hypotension
- Decreased mucociliary clearance (horses)
- Cardiovascular depression (bradycardia, arrhythmias)
- Vomiting (especially in cats)
- Diuresis (suppression of ADH secretion)
Drug interactions
- Acepromazine ยท Potential for additive hypotensive effects; use cautiously in animals susceptible to hemodynamic complications.
- Chloramphenicol ยท Prolonged sedation and gastrointestinal stasis possible.
- CNS Depressants (barbiturates, narcotics, anesthetics, phenothiazines) ยท Additive CNS depression. Dosages of these agents may need to be reduced.
- Epinephrine ยท May induce the development of ventricular arrhythmias (with or without concurrent halothane).
- Reserpine ยท May cause colic-like clinical signs in horses; concurrent use should be avoided.
- Anaesthetic agents ยท Significantly reduces the dose of all other anaesthetic agents required to maintain anaesthesia. ยท major
- Sympathomimetic amines ยท Increased risk of severe cardiac arrhythmias. ยท major
Monitoring
- Level of anesthesia/analgesia
- Respiratory function (rate, depth, tidal volume)
- Cardiovascular status (heart rate, rhythm, blood pressure)
- Hydration status (especially if polyuria is present)
- Heart rate and rhythm (ECG)
- Blood pressure
- Respiratory rate and depth
- Blood glucose (in susceptible patients)
Overdose
In the event of an accidental overdosage, profound **CNS and respiratory depression**, **hypotension**, and **cardiac arrhythmias** may occur. Seizures have also been reported. * **Reversal Agents:** Alpha-2 antagonists such as **yohimbine**, **atipamezole**, and **tolazoline** can be used alone or in combination to reverse the effects of xylazine and speed recovery times. * **Supportive Care:** Mechanical respiratory support and respiratory stimulants (e.g., doxapram) may be required to treat severe respiratory depressant effects.
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.