Phenoxybenzamine
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: Phenoxybenzamine covalently and irreversibly binds to **alpha-1 and alpha-2 adrenergic receptors** โ non-competitive blockade of circulating epinephrine and norepinephrine โ profound vasodilation and relaxation of smooth muscle. - **Urinary Tract**: Blockade of alpha receptors in the trigone and internal urethral sphincter โ decreased urethral resistance โ facilitation of bladder emptying. - **Cardiovascular**: Blockade of vascular alpha receptors โ decreased peripheral vascular resistance โ 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.
Dosing by species
- Functional urethral obstruction ยท 2.5-7.5 mg/cat ยท PO ยท once to twice daily
- Functional urethral obstruction ยท 1.25-7.5 mg (total dose) ยท PO ยท q12-24h
- Functional urethral obstruction ยท 2.5-10 mg (total dose) ยท PO ยท q24h
- Short-term treatment of hypertension ยท 0.5 mg/kg ยท PO ยท q12h
- Short-term treatment of hypertension ยท 2.5 mg (total dose) increasing by 2.5 mg up to a maximum of 10 mg (total dose) ยท PO ยท q12h
- Short-term treatment of hypertension ยท 2.5-7.5 mg per cat ยท PO ยท q8-12h
- Decrease urethral sphincter tone in bladder paresis ยท 0.7 mg/kg ยท PO ยท 4 times a day ยท Used in combination with bethanechol at 0.25-0.75 mg/kg PO 2-4 times a day.
- Adjunctive treatment of laminitis (developmental phase) ยท 1 mg/kg ยท IV ยท q12h ยท for 2 doses
- Treatment of profuse, watery diarrhea ยท 200-600 mg ยท q12h
- Functional urethral obstruction ยท 0.25 mg/kg ยท PO ยท q12-24h ยท Alternative dose: 2.5-20 mg (total dose) PO q12-24h
Routes of administration
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)
- Hypotension
- Tachycardia
Drug interactions
- Epinephrine ยท 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.
- Phenylephrine ยท Phenoxybenzamine will antagonize the effects of alpha-adrenergic sympathomimetic agents.
- Reserpine ยท Phenoxybenzamine can antagonize the hypothermic effects of reserpine.
- Beta-blockers ยท 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. ยท major
- Diuretics ยท Increased risk of first dose hypotensive effect. ยท moderate
- Alpha-adrenergic sympathomimetics (e.g., phenylephrine) ยท Phenoxybenzamine will antagonize the effects of these agents. ยท major
Monitoring
- Clinical efficacy (e.g., adequate urination, ease of voiding)
- Blood pressure (monitor for hypotension)
- Heart rate (monitor for reflex tachycardia)
- Blood pressure
- Heart rate and rhythm
- Urine output and ease of urination
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.
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.