What are the side effects of sildenafil (Viagra)?

Sildenafil Side Effects

Sildenafil causes primarily mild to moderate vasodilatory side effects, with headache, flushing, nasal congestion, and dyspepsia being most common, and carries an absolute contraindication with nitrates due to risk of life-threatening hypotension. 1, 2

Common Vasodilatory Side Effects

The most frequently reported adverse effects are directly related to sildenafil's mechanism of phosphodiesterase type-5 inhibition and resulting vasodilation:

• Headache is the most common side effect across all clinical trials, occurring in approximately 25-30% of patients 3, 1, 4
• Flushing (facial redness/warmth) occurs in approximately 31% of patients and is transient 1, 4
• Nasal congestion and epistaxis (nosebleeds) are common vasodilatory effects reported in 19% of patients 3, 1, 4
• Dyspepsia (upset stomach/epigastric discomfort) occurs in approximately 10-11% of patients 1, 5, 4
• Abnormal vision including blue-tinted vision or increased light sensitivity occurs in some patients 1, 5

Dose-Related Pattern

• Side effects demonstrate a clear dose-response relationship, with higher doses (40-80 mg three times daily) producing more frequent adverse effects than the FDA-approved 20 mg three times daily dose 4
• Most adverse effects are transient, mild to moderate in severity, with only approximately 3-4% of patients discontinuing treatment due to side effects 3, 5

Serious Cardiovascular Effects

• Hypotension can occur, particularly transient decreases in systolic/diastolic blood pressure of 7-9/6-7 mm Hg with therapeutic doses 6
• Absolute contraindication with nitrates (nitroglycerin, isosorbide dinitrate) exists due to synergistic potentiation of vasodilation causing potentially life-threatening hypotension 1, 2, 5
• Modest decreases in systemic vascular resistance (up to 16%) may occur without significant heart rate changes 6
• Patients may experience increased shortness of breath, which should be reported immediately as it may indicate worsening underlying pulmonary arterial hypertension 2

Serious Ocular and Auditory Effects

• Non-arteritic anterior ischemic optic neuropathy (NAION): Very-low-quality evidence suggests PDE-5 inhibitors may be associated with increased risk of "possible NAION," though not definitively proven 1
• Sudden vision loss in one or both eyes requires immediate medical attention and drug discontinuation 2
• Sudden hearing loss or decrease has been reported and requires immediate physician consultation, though causality is not definitively established 2

Rare but Critical Side Effects

• Priapism (erections lasting >4 hours) is rare but requires immediate medical attention to prevent permanent penile damage 1, 2
• Cardiovascular events including heart attack, stroke, irregular heartbeats, and death have been reported, though most occurred in men with pre-existing heart disease 2

Additional Side Effects in Pulmonary Arterial Hypertension

When used for PAH treatment, additional adverse effects include:

• Fever, respiratory infection, bronchitis, pharyngitis, runny nose, and pneumonia (particularly in pediatric populations) 2
• Nausea and vomiting 2
• Trouble sleeping 2

Critical Drug Interactions

• Riociguat: Absolute contraindication due to hypotension risk 1
• CYP3A4 inhibitors (ritonavir, ketoconazole, erythromycin, cimetidine) require dose reduction to 25 mg due to increased sildenafil plasma levels 2, 5
• Warfarin: No direct interaction with sildenafil, though other endothelin receptor antagonists used in PAH may require warfarin dose adjustment 3

Common Pitfall to Avoid

• Do not assume all side effects are benign—sudden vision or hearing loss, priapism, and severe hypotension require immediate medical evaluation despite the overall favorable safety profile 1, 2
• The incidence of adverse events may be higher in real-world practice (31.6%) than initially reported in industry-sponsored trials, likely due to more explicit questioning about side effects 4