Sildenafil
Sildenafil citrate
Also known as: Viagra · Revatio · Aphrodil · UK 92480-10 · Sildenafil citrate
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 |
|---|---|---|---|---|---|
| Pulmonary hypertension | 1.9 mg/kg (range from 0.5-2.7 mg/kg) | PO | q8-24h | — | 🌎 NA |
| Pulmonary hypertension documented by Doppler, chronic pulmonary disease, right-sided heart failure (HW disease; congenital) | 0.5-1 mg/kg (higher dose of 2 -3 mg/kg three times a day may be tolerated and needed) | PO | two times daily | — | 🌎 NA |
| Moderate to severe PH secondary to either left heart disease or primary lung disease | 1 mg/kg | PO | q8h | — | 🌎 NA |
| Pulmonary hypertension | 1-2 mg/kg | PO | three times daily | — | 🌎 NA |
| Pulmonary arterial hypertension | 0.5-2.7 mg/kg (suggested median dose from clinical studies is 3 mg/kg/day) | PO | q8-24h | — | 🌍 EU |
- Pulmonary hypertension: Median dose from a retrospective study. Dogs may have been also treated with oxygen, ACE inhibitors, furosemide, amlodipine, diltiazem, theophylline, phenobarbital and/or antibiotics.
- Pulmonary arterial hypertension: Improves quality of life despite lack of significant echocardiographic reduction in pressures.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Moderate to severe PH secondary to either left heart disease or primary lung disease | 1 mg/kg | PO | q8h | — | 🌎 NA |
| Pulmonary arterial hypertension | 1.6 mg/kg | PO | q12h | — | 🌍 EU |
- Pulmonary arterial hypertension: Reported dose tolerated and clinically efficacious.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Sildenafil is a potent phosphodiesterase type 5 (PDE5) inhibitor. While primarily recognized in human medicine for the treatment of erectile dysfunction, in veterinary pharmacology, it is a cornerstone therapy for pulmonary hypertension (PH).
- Clinical Pearl: It is particularly effective for dogs and cats suffering from PH secondary to chronic mitral valve disease, heartworm disease, or primary pulmonary diseases (such as pulmonary fibrosis).
- By selectively dilating the pulmonary arterial bed, sildenafil significantly reduces right ventricular afterload, thereby improving clinical signs like syncope (fainting), exercise intolerance, and right-sided congestive heart failure.
- It is highly specific to the pulmonary vasculature, meaning it typically does not cause severe systemic hypotension unless combined with other vasodilators.
Mechanism of action
Sildenafil exerts its effects via the nitric oxide (NO) pathway in smooth muscle cells:
- Endothelial cells release Nitric Oxide (NO) → activates the enzyme guanylate cyclase → converts GTP to cyclic guanosine monophosphate (cGMP).
- cGMP acts as a secondary messenger causing smooth muscle relaxation and vasodilation.
- PDE5 is the enzyme responsible for degrading cGMP, and it is found in high concentrations in the pulmonary vasculature and corpus cavernosum.
- Mechanism: Sildenafil competitively inhibits PDE5 → prevents cGMP breakdown → intracellular cGMP accumulates → resulting in prolonged and enhanced NO-mediated pulmonary vasodilation.
Safety & warnings
Contraindications
- Concurrent use with organic nitrates
- Known hypersensitivity to sildenafil
- Pulmonary veno-occlusive disease (PVOD)
- Systemic hypotension
- Significant hepatic impairment
- Significant renal impairment
- Bleeding disorders
- Concurrent use of nitrates
Adverse effects
- Cutaneous flushing of the inguinal region
- Gastrointestinal effects (possible)
- Headache, visual disturbances, dyspepsia, nasal congestion, myalgia, priapism, dizziness, and back pain (reported in humans)
- Vomiting
- Dizziness / weakness
- Raised intraocular pressure
- Systemic hypotension (especially if combined with other vasodilators)
Precautions
WARNING: Sildenafil should not be used concurrently with nitrates due to the risk of life-threatening hypotension.
- Pulmonary vasodilators may significantly worsen the cardiovascular status of patients with pulmonary veno-occlusive disease (PVOD).
- Use with extreme caution in patients with resting hypotension, fluid depletion, severe left ventricular outflow obstruction, or autonomic dysfunction.
Drug interactions
May increase hypotensive effects
Potential to increase hypotensive effects
Potentially could increase hypotensive effects
May reduce sildenafil metabolism and increase AUC
May reduce sildenafil metabolism and increase AUC
May reduce sildenafil metabolism and increase AUC
May increase bleeding risks
Significant potentiation of vasodilatory effects; life-threatening hypotension possible
Significant potentiation of vasodilatory effects; life-threatening hypotension possible
May decrease sildenafil concentrations
May decrease sildenafil concentrations
Increases plasma sildenafil concentration
Increases plasma sildenafil concentration
Increased risk of systemic hypotension; should not be started at the same time
Significantly enhances hypotensive effect; concomitant use must be avoided
Monitoring
- Clinical efficacy (improved syncope, cough, respiratory effort)
- Pulmonary artery pressure
- Systemic blood pressure
- Adverse effects
- Clinical signs of pulmonary hypertension (exercise tolerance, syncope, respiratory rate/effort)
- Echocardiography (to monitor right heart function and pulmonary pressures)
- Intraocular pressure (if pre-existing ocular disease)
Pharmacokinetics
Half-life
Absorption
Oral bioavailability is approximately 50% in dogs (higher than humans).
Distribution
Volume of distribution is about 5.2 L/kg in dogs (versus 1.2 L/kg in humans).
Metabolism
Hepatic metabolism (implied by interactions with CYP inhibitors/inducers).
Elimination
Excreted primarily as metabolites.
Overdose
Little information is available in veterinary species. In humans, massive overdose (2000 mg) resulted in tachycardia, nonspecific ST-T changes on ECG, headache, dizziness, and flushing.
It is expected that overdoses in animals would mirror the drug's adverse effect profile (e.g., profound hypotension, tachycardia, cutaneous flushing). Treat supportively.
Available products
Formulations
- Oral tablets
- Injection solution
- 20 mg tablet
- 50 mg tablet
- 6.25 mg tablet (special reformulation)
- 10 mg tablet (special reformulation)
- 12.5 mg tablet (special reformulation)
- 10 mg/ml oral suspension
- 0.08 mg/ml injectable (IV)
Veterinary
- Special reformulations available: 6.25 mg, 10 mg and 12.5 mg tablets
Human-labeled
- Sildenafil Citrate Oral Tablets: 20 mg Revatio® (Pfizer); 25 mg, 50 mg & 100 mg Viagra® (Pfizer)
- Sildenafil Injection Solution: 10 mg/12.5 mL (dextrose 50.5 mg/mL) in 12.5 mL single-use vials; Revatio® (Pfizer)
- Revatio (20 mg tablets, 10 mg/ml oral suspension, 0.08 mg/ml IV)
- Viagra (25 mg, 50 mg tablets)
Regulatory status
POM (Prescription Only Medicine). Used off-label under the cascade.
POM-V / POM. Used off-label under the prescribing cascade.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Sildenafil tablets should be stored at room temperature (25°C; 77°F); excursions permitted to 15-30°C (59-86°F).
Client information
Important: Sildenafil is often prescribed "off-label" in veterinary medicine to treat high blood pressure in the lungs (pulmonary hypertension).
- What to expect: You may notice your pet has more energy, less coughing, and fewer fainting spells (syncope).
- Cost: Because this is a human medication (often known as Viagra® or Revatio®), it can be expensive. Discuss compounding or generic options with your veterinarian if cost is a concern.
- Safety Warning: NEVER give your pet any other heart medications, especially nitrates (like nitroglycerin), while they are on sildenafil, as this combination can cause a fatal drop in blood pressure.
- Side Effects: Watch for redness or flushing on the belly (inguinal area) or stomach upset. Contact your vet if your pet seems dizzy, lethargic, or collapses.
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.