Prazosin

Prazosin is an alpha1-adrenergic antagonist that relaxes vascular and urethral smooth muscle. Veterinary use includes selected functional urinary-outflow disorders and empirical adjunctive vasodilation. Feline use after relief of urethral obstruction is well documented, but controlled studies have not established a reduction in recurrent obstruction.

Group

Alpha1-adrenergic antagonist

Mechanism of Action

Prazosin competitively blocks alpha1-adrenergic receptors on smooth muscle. Reduced adrenergic stimulation relaxes arterial and venous vessels and decreases tone in the smooth-muscle portion of the urethra. These effects provide the rationale for vasodilation and selected urinary applications. It does not directly relax striated urethral-sphincter muscle, which has different neural control.

Dose Rates

Cats

Use / stage	Dose	Route	Schedule	Instructions
Urethral relaxation or vasodilation	0.25–1 mg/animal	PO	Every 8–12 h
Urethral relaxation or vasodilation	0.07 mg/kg	PO	Every 8–12 h
Urethral obstruction adjunct — Initial	0.5 mg/animal	PO	Every 8 h
Urethral obstruction adjunct — Maintenance	0.25–0.5 mg/animal	PO	Every 24 h
Urethral obstruction recurrence prevention	0.25 mg/animal	PO	Every 12 h	Up to 30 days; Controlled study found no advantage over placebo at this lower regimen; retained as an ineffective reported alternative.
Urethral relaxation	0.03 mg/kg	IV
Functional urethral obstruction	0.25–0.5 mg/animal	PO	Every 12–24 h
Urethral spasm	0.25–1 mg/animal	PO	Every 8–12 h	Up to 7 days; Initial 5-7 days, then wean if possible.

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Heart failure adjunct	1 mg/animal	PO	Every 8 h	Dogs under 15 kg. Exact 15-kg boundary is unspecified.
Heart failure adjunct	2 mg/animal	PO	Every 8 h	Dogs over 15 kg. Exact 15-kg boundary is unspecified.
Hypertension	1–4 mg/animal	PO	Every 12–24 h
Urethral relaxation	0.0666666667 mg/kg	PO	Every 8 h
Functional urethral obstruction	0.0666666667 mg/kg	PO	Every 8–24 h
Urethral relaxation	0.0666666667 mg/kg	PO	Every 12–24 h

Indications

Empirical treatment of selected functional urethral outflow resistance or urethral smooth-muscle spasm in dogs, with limited clinical efficacy evidence.
Empirical urethral relaxation after relief of feline urethral obstruction; controlled studies have not demonstrated prevention of recurrent obstruction.
Adjunctive vasodilation in selected dogs with congestive heart failure or systemic hypertension, with limited outcome evidence.

Contraindications

Compromised cardiac function or hypotension requires caution.

Interactions

Other vasodilators can cause marked hypotension.

Adverse effects

Hypotension; feline lethargy, ptyalism and diarrhea.

Toxicity

Excess alpha blockade can cause hypotension and reflex tachycardia; stabilize circulation before decontamination and monitor renal perfusion.

Preparations

Preparation	Form	Strength	Route
Prazosin capsule 1 mg	Capsule	1 mg / capsule	PO
Prazosin capsule 2 mg	Capsule	2 mg / capsule	PO
Prazosin capsule 5 mg	Capsule	5 mg / capsule	PO

Monitoring and Administration

Titrate against response and monitor blood pressure and reflex tachycardia. Evidence for veterinary indications is limited.

Dogs: 0.5-2 mg/dog every 8-24 hours is reported, but this canine regimen does not specify route.