Clonidine
Clonidine is a **central alpha-2 adrenergic agonist** traditionally used in veterinary medicine as a diagnostic agent for growth hormone (GH) deficiency or pheochromocytoma in dogs. It also serves as an adjunctive treatment for refractory **inflammatory bowel disease (IBD)**, particularly in cats, and as an epidural adjunct for severe pain or surgical anesthesia. > **Clinical Pearls (Pharmacology Enrichment):** > * While the monograph highlights its diagnostic and gastrointestinal uses, clonidine is increasingly utilized off-label in modern veterinary behavioral medicine to manage **fear-based anxiety, noise phobias, and situational panic** in dogs. > * It provides anxiolysis and sedation by dampening sympathetic arousal, often without the profound ataxia seen with other sedatives. > * Because of its cardiovascular effects, it requires careful patient selection and dose titration.
Mechanism: Clonidine acts centrally at the **brain stem**, stimulating **alpha-2 adrenoreceptors**. * **Cardiovascular/Sympathetic:** Stimulation of presynaptic **alpha-2 receptors** (e.g., in the **locus coeruleus**) โ decreases norepinephrine release โ reduces sympathetic outflow from the CNS โ decreased peripheral resistance, heart rate, and blood pressure. * **Endocrine:** Stimulates **GHRH (Growth Hormone Releasing Hormone)** โ transient release of **Growth Hormone (GH)**. * **Analgesia:** Stimulates presynaptic and postjunctional **alpha-2 adrenoreceptors** in the dorsal horn of the spinal cord โ blocks pain signal transmission to the brain. * **Gastrointestinal:** Alpha-2 stimulation alters enteric nervous system tone, increasing fluid absorption and decreasing motility, aiding in refractory IBD.
Dosing by species
- Adjunctive antidiarrheal therapy for refractory cases of inflammatory bowel disease ยท 5-10 micrograms/kg ยท SC or PO ยท two to three times a day ยท As fourth line therapy after prostaglandin synthetase inhibitors, opioid agonists, and 5-HT3 serotonergic antagonists are being used.
- Epidural analgesia/analgesia ยท 2-3 micrograms/kg diluted to 8 mL with sterile normal saline ยท epidurally ยท Single dose ยท Onset/duration of analgesia = 19 minutes/192 minutes with 2 mcg/kg dose and = 9 minutes/311 minutes with 3 mcg/kg dose; peak effects from 60-180 minutes.
- Diagnosing hyposomatotropism ยท 10 micrograms/kg ยท IV ยท Single dose ยท Obtain plasma for GH levels prior to dosing and at 15, 30, 45, 60, and 120 minutes. Healthy dogs should demonstrate GH levels of 10 ng/mL after administration.
- Adjunctive antidiarrheal therapy for refractory cases of inflammatory bowel disease ยท 5-10 micrograms/kg ยท PO or SC ยท two to three times a day ยท Can activate alpha2-receptors in the CRT and cause vomiting.
- Control panic-like responses and fear-based behavioural problems ยท Dose not specified in monograph; use tactically (prn) or over longer term ยท PO ยท prn or daily ยท prn (lasts 3-4 hours) or long-term (takes 1-2 weeks for full response) ยท Effect develops in about 30 minutes. Withdrawal must be gradual.
- Diagnostic test for pituitary dwarfism (hyposomatotropism) ยท Dose not specified in monograph ยท IV ยท Single dose ยท N/A ยท Specialist texts should be consulted for the clonidine stimulation test protocol.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Known hypersensitivity to clonidine
- Severe cardiovascular disease (use with extreme caution)
- Renal disease (use with extreme caution)
Adverse effects
- Transient hyperglycemia
- Dry mouth
- Constipation
- Sedation
- Aggressive behavior
- Hypotension
- Collapse
- Bradycardia
- Transient sedation
- Rebound hypertension (if withdrawn abruptly)
Drug interactions
- Antihypertensive drugs ยท Possible additive hypotensive effects
- Beta-adrenergic blocking agents (e.g., propranolol) ยท May enhance bradycardia. If discontinuing, beta-blocker should be stopped first, then clonidine tapered to avoid rebound hypertension.
- CNS depressant drugs (opiates, barbiturates) ยท May exacerbate the actions of other CNS depressant drugs
- Digoxin ยท Possible additive bradycardia
- Prazosin ยท May decrease the antihypertensive effects of clonidine
- Tricyclic antidepressants (e.g., amitriptyline) ยท May block the antihypertensive effects of clonidine
- Barbiturates ยท Increased central nervous system depression and hypotension ยท major
- Opiates ยท Additive sedation, bradycardia, and respiratory depression ยท major
- Beta-blockers ยท Additive hypotensive and bradycardic effects; may exacerbate rebound hypertension upon withdrawal ยท major
- Hypotensive agents ยท Additive lowering of blood pressure ยท moderate
Monitoring
- Growth Hormone (GH) levels and adverse effects (when used for diagnostics)
- Blood pressure
- Heart rate
- Efficacy and adverse effect profile (when used for ongoing diarrhea treatment)
- Degree of sedation
- Behavioral response
- Plasma IGF-1 or GH levels (when used diagnostically)
Overdose
Clonidine has a **narrow margin of safety**. * **Clinical Signs:** Hypotension, bradycardia, vomiting, weakness, depression, lethargy, tachycardia, ataxia, and hypertension. Rarely, seizures or respiratory depression. * **Toxicity Values:** LD50 for oral clonidine in rats is 465 mg/kg; mice 206 mg/kg. * **Treatment:** For large overdoses, gut evacuation using standard protocols (use emetics cautiously as consciousness may deteriorate rapidly). Symptomatic and supportive care. Fluids and pressors (e.g., dopamine) for hypotension. IV atropine for bradycardia. **Atipamezole or yohimbine** may be used to reverse cardiovascular effects, but multiple doses may be required as signs can last up to 48 hours.
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.