Sildenafil

Sildenafil is a phosphodiesterase-5 inhibitor used principally to reduce pulmonary vascular resistance in selected veterinary patients with pulmonary hypertension. Its smooth-muscle effects also support a more specialized application in some dogs with megaesophagus. Clinical response depends on the underlying pulmonary vascular or esophageal disorder and is not uniform across patient groups.

Group

Phosphodiesterase V inhibitor

Mechanism of Action

Sildenafil inhibits breakdown of cyclic GMP by phosphodiesterase-5, prolonging nitric-oxide-associated smooth-muscle relaxation. In pulmonary vessels, this can lower vascular resistance and improve right-ventricular loading conditions. At the lower esophageal sphincter, relaxation can facilitate passage of esophageal contents into the stomach in selected patients. The effect depends on existing signaling and the tissue involved.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Pulmonary hypertension	2 mg/kg	PO	Every 12 h
Pulmonary hypertension	2 mg/kg	PO	Every 8–24 h	Alternative reported intervals individualized to response.
Pulmonary hypertension	3 mg/kg	PO	Every 8–24 h	Higher dose reported in selected dogs.
Megaesophagus	1 mg/kg	PO	Every 12 h
Pulmonary hypertension	0.5–1 mg/kg	PO	Every 12 h
Moderate to severe pulmonary hypertension	1 mg/kg	PO	Every 8 h
Pulmonary hypertension	1–2 mg/kg	PO	Every 8 h
Pulmonary hypertension higher-dose alternative	2–3 mg/kg	PO	Every 8 h	Selected patients; titrate with cardiovascular assessment.

Cats

Use / stage	Dose	Route	Schedule
Reported sildenafil therapy	1 mg/kg	PO	Every 8 h
Pulmonary hypertension	0.5–1 mg/kg	PO	Every 12 h
Moderate to severe pulmonary hypertension	1 mg/kg	PO	Every 8 h

Indications

Treatment of clinically significant pulmonary hypertension in dogs with a responsive pulmonary vascular component.
Specialist treatment of selected pulmonary hypertension in cats or other veterinary species, with a smaller evidence base.
Adjunctive treatment of selected congenital or generalized canine megaesophagus, with inconsistent results across clinical studies.

Contraindications

Other vasodilators can increase hypotension.

Interactions

Nitrates and alpha blockers can markedly reduce blood pressure.
Cisapride and metoclopramide oppose the intended effect in megaesophagus.

Adverse effects

Flushing and potential hypotension.

Toxicity

Excess vasodilation may cause hypotension or tachycardia; animal overdose data are limited and management is supportive.

Preparations

Preparation	Form	Strength	Route
Sildenafil 20 mg	Tablet	20 mg / tablet	PO
Sildenafil 25 mg	Tablet	25 mg / tablet	PO
Sildenafil 50 mg	Tablet	50 mg / tablet	PO
Sildenafil 100 mg	Tablet	100 mg / tablet	PO

Monitoring and Administration

Assess cardiovascular response and blood pressure in patients at risk; evidence is mainly from canine studies.

A retrospective canine series reported median 1.9 mg/kg and range 0.5-2.7 mg/kg every 8-24 hours without specifying route in that dose line; it is not a separate oral instruction by assumption.