Phenoxybenzamine
Phenoxybenzamine HCl
Also known as: Dibenzyline · Dibenyline · SKF-688A · Dibenzyran · Fenoxene · Phenoxybenzaminum
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Functional urethral obstruction | 0.25 mg/kg | PO | q12-24h | — | 🌎 NA |
| Functional urethral obstruction | 0.25 mg/kg | PO | q12h | — | 🌎 NA |
| Functional urethral obstruction | 0.25-0.5 mg/kg | PO | once or twice daily | — | 🌎 NA |
| Functional urethral obstruction | 5-15 mg (total dose) | PO | q12h | — | 🌎 NA |
| Hypertension associated with pheochromocytoma (pre-surgical) | Initially 0.5 mg/kg, gradually increase every few days until clinical signs of hypotension, adverse reactions, or max dosage of 2.5 mg/kg is attained | PO | twice daily | 1-2 weeks prior to surgery | 🌎 NA |
| Hypertension associated with pheochromocytoma (medical management) | Initial dose 0.25 mg/kg, gradually increase every few days until signs of hypotension, adverse reaction, or max dosage of 2.5 mg/kg is attained | PO | twice daily | Long-term | 🌎 NA |
| Adjunctive treatment of endotoxicosis | 0.25-0.5 mg/kg | PO | q6h | — | 🌎 NA |
- Functional urethral obstruction: Alternative dose: 2.5-20 mg (total dose) PO q12-24h
- Hypertension associated with pheochromocytoma (pre-surgical): Continue until the time of surgery. Close perioperative monitoring is critical.
- Hypertension associated with pheochromocytoma (medical management): Beta-blockers may be added for severe tachycardia, but NEVER without prior alpha-blockade to avoid severe hypertension.
- Adjunctive treatment of endotoxicosis: Use with appropriate antimicrobials, steroids, and supportive care.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Functional urethral obstruction | 2.5-7.5 mg/cat | PO | once to twice daily | — | 🌎 NA |
| Functional urethral obstruction | 1.25-7.5 mg (total dose) | PO | q12-24h | — | 🌎 NA |
| Functional urethral obstruction | 2.5-10 mg (total dose) | PO | q24h | — | 🌎 NA |
| Short-term treatment of hypertension | 0.5 mg/kg | PO | q12h | — | 🌎 NA |
| Short-term treatment of hypertension | 2.5 mg (total dose) increasing by 2.5 mg up to a maximum of 10 mg (total dose) | PO | q12h | — | 🌎 NA |
| Short-term treatment of hypertension | 2.5-7.5 mg per cat | PO | q8-12h | — | 🌎 NA |
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Decrease urethral sphincter tone in bladder paresis | 0.7 mg/kg | PO | 4 times a day | — | 🌎 NA |
| Adjunctive treatment of laminitis (developmental phase) | 1 mg/kg | IV | q12h | for 2 doses | 🌎 NA |
| Treatment of profuse, watery diarrhea | 200-600 mg | — | q12h | — | 🌎 NA |
- Decrease urethral sphincter tone in bladder paresis: Used in combination with bethanechol at 0.25-0.75 mg/kg PO 2-4 times a day.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Phenoxybenzamine is a long-acting, non-competitive alpha-adrenergic antagonist.
In veterinary medicine, it is primarily utilized for:
- Small Animals: Reducing internal urethral sphincter tone to treat functional urethral obstruction or detrusor areflexia. It is also a critical pre-operative medication for patients with pheochromocytoma to prevent life-threatening hypertensive crises during surgical manipulation of the tumor.
- Horses: Niche applications including the management of early-stage laminitis, secretory diarrheas, and bladder paresis.
Clinical Pearl: Because phenoxybenzamine binds irreversibly to receptors, its effects are prolonged and can only be reversed by the synthesis of new receptors, a process that takes several days.
Mechanism of action
Phenoxybenzamine covalently and irreversibly binds to alpha-1 and alpha-2 adrenergic receptors → non-competitive blockade of circulating epinephrine and norepinephrine → profound vasodilation and relaxation of smooth muscle.
- Urinary Tract: Blockade of alpha receptors in the trigone and internal urethral sphincter → decreased urethral resistance → facilitation of bladder emptying.
- Cardiovascular: Blockade of vascular alpha receptors → decreased peripheral vascular resistance → lowered blood pressure (often causing reflex tachycardia).
Because the binding is irreversible, the effect is often described as a 'chemical sympathectomy'. It has no effect on beta-adrenergic or parasympathetic receptors.
Safety & warnings
Contraindications
- Conditions where hypotension is undesirable (e.g., shock, unless fluid replacement is adequate)
- Horses with clinical signs of colic
- Glaucoma (relative contraindication)
- Diabetes mellitus (relative contraindication)
Adverse effects
- Hypotension (postural/orthostatic)
- Hypertension (rebound)
- Miosis
- Increased intraocular pressure
- Tachycardia (reflex)
- Sodium retention
- Inhibition of ejaculation
- Nasal congestion
- Weakness or dizziness
- Gastrointestinal effects (nausea, vomiting)
- Constipation (specifically noted in horses)
Precautions
Use with extreme caution in patients with congestive heart failure (CHF) or other heart disease, as drug-induced tachycardia can occur. Use cautiously in patients with renal damage or cerebral/coronary arteriosclerosis.
Pregnancy: FDA Category C. Animal studies (guinea pigs) have shown abnormalities in the closure of the patent ductus. Use only if benefits outweigh risks.
Drug interactions
Epinephrine reversal: If used with drugs that have both alpha- and beta-adrenergic effects, increased hypotension, vasodilation, or tachycardia may result due to unopposed beta effects.
Phenoxybenzamine will antagonize the effects of alpha-adrenergic sympathomimetic agents.
Phenoxybenzamine can antagonize the hypothermic effects of reserpine.
Increased risk of first dose hypotensive effect; if used for phaeochromocytoma, beta-blockers must only be started after alpha blockade is in place to avoid hypertensive crisis.
Increased risk of first dose hypotensive effect.
Phenoxybenzamine will antagonize the effects of these agents.
Monitoring
- Clinical efficacy (e.g., adequate urination, ease of voiding)
- Blood pressure (monitor for hypotension)
- Heart rate (monitor for reflex tachycardia)
- Heart rate and rhythm
- Urine output and ease of urination
Pharmacokinetics
Absorption
Variably absorbed from the GI tract, with a bioavailability of 20-30% in humans. Onset of action is slow (several hours) and increases over several days after regular dosing. Effects persist for 3-4 days after discontinuation.
Distribution
Highly lipid soluble and may accumulate in body fat. Unknown if it crosses the placenta or enters milk.
Metabolism
Metabolized (dealkylated) in the liver.
Elimination
Excreted in both the urine and bile.
Overdose
Overdosage may yield signs of postural hypotension (dizziness, syncope), tachycardia, vomiting, lethargy, or shock.
Treatment:
- Empty the gut if ingestion was recent and there are no contraindications.
- Treat hypotension with aggressive intravenous fluid support.
- DO NOT USE EPINEPHRINE (causes paradoxical further drop in blood pressure due to unopposed beta-adrenergic vasodilation).
- Most standard vasopressors are ineffective due to the alpha-blockade. Intravenous norepinephrine (levarterenol) may be beneficial if clinical signs are severe.
Available products
Formulations
- Capsules
- 10 mg oral capsule
- 50 mg/ml injectable solution
Human-labeled
- Phenoxybenzamine HCl Capsules: 10 mg (Dibenzyline)
- Dibenyline 10 mg capsules
- Phenoxybenzamine 50 mg/ml solution
Regulatory status
POM (Prescription Only Medicine)
POM (Prescription Only Medicine)
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Capsules should be stored at room temperature in well-closed containers.
Client information
Phenoxybenzamine is often prescribed to help your pet urinate more easily by relaxing the muscles of the urinary tract, or to manage high blood pressure associated with certain adrenal tumors.
- Patience is key: It may take a week or longer to see the full effect on your pet's urination. Do not stop the medication early unless directed by your veterinarian.
- Give with food: Administering this medication with meals can help reduce stomach upset (nausea or vomiting).
- Watch for low blood pressure: Because this drug relaxes blood vessels, it can cause a drop in blood pressure. Contact your veterinarian immediately if your pet appears weak, dizzy, or collapses, especially after standing up.
- Missed doses: If you miss a dose, give it as soon as you remember, but never give two doses at once.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.