Side effects of sildenafil
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Here is a complete breakdown of sildenafil's side effects based on authoritative pharmacology sources:
Side Effects of Sildenafil
Mechanism Underlying Side Effects
Sildenafil is a selective PDE5 inhibitor that elevates cGMP levels, augmenting vasodilation. Most adverse effects arise directly from this mechanism - non-specific vasodilation and, to a lesser extent, inhibition of related phosphodiesterase isoforms (particularly PDE6 in the retina).
Common Side Effects
| Side Effect | Frequency | Mechanism |
|---|---|---|
| Headache | ~16% | Cerebral vasodilation |
| Flushing | ~10% | Peripheral vasodilation |
| Nasal congestion | Common | Vasodilation of nasal mucosa |
| Dyspepsia / indigestion | Common | Smooth muscle relaxation in GI tract |
| Diarrhea | Common | GI smooth muscle effects |
| Rash | Common | - |
| Dizziness | Common | Vasodilation/hypotension |
| Blue-green tinting of vision ("blue vision") | Common | Inhibition of retinal PDE6 (involved in phototransduction) - seen with sildenafil and vardenafil |
| Abnormal vision / transient color discrimination impairment | Common | PDE6 inhibition |
- Goodman & Gilman's Pharmacological Basis of Therapeutics: "Headache (16%) and flushing (10%) are the most frequently reported side effects. Patients taking sildenafil or vardenafil may notice a transient blue-green tinting of vision due to inhibition of retinal PDE6."
Serious / Rare Side Effects
- Severe hypotension - The most dangerous interaction. Sildenafil potentiates the hypotensive effects of nitrate vasodilators (e.g., nitroglycerin), producing dangerously low blood pressure. Concurrent use with nitrates in any form is contraindicated.
- Priapism - Prolonged, painful erection; a urologic emergency requiring prompt treatment.
- Permanent vision loss (NAION) - Non-arteritic ischemic optic neuropathy; rare but serious. Risk is higher in patients with pre-existing cardiovascular risk factors.
- Hearing impairment / sudden hearing loss - Rare, reported post-marketing.
- Vaso-occlusive crises - In patients with sickle cell disease; use is not recommended in this population.
Special Population Concerns
- Children / Pediatric PAH: Chronic use has been associated with increased mortality. Hazard ratio for mortality was 3.95 for high vs. low doses; long-term use (>=2 years) carries significant mortality risk.
- Elderly (>65 years): Reduced clearance leads to increased drug exposure (higher AUC); dose adjustment may be needed.
- Severe hepatic/renal impairment (GFR <30 mL/min): Significantly reduced clearance; use with caution.
- Sepsis: High cGMP levels may potentiate hypotension.
Drug Interactions Causing Increased Toxicity
The following drugs inhibit CYP3A4/2C9 and raise sildenafil levels, increasing side effect risk:
- Azole antifungals (e.g., ketoconazole)
- Macrolides (erythromycin, clarithromycin)
- HIV protease inhibitors
- Cimetidine, ciprofloxacin, nicardipine, verapamil, quinidine
- Grapefruit juice
Drugs that decrease sildenafil efficacy (CYP inducers): bosentan, rifampin, carbamazepine, phenytoin, phenobarbital, St. John's wort, efavirenz.
Summary by System
| System | Effect |
|---|---|
| Cardiovascular | Hypotension, flushing, palpitations |
| CNS | Headache, dizziness |
| Visual | Blue-green tinge, color discrimination loss, NAION (rare) |
| Auditory | Sudden hearing loss (rare) |
| GI | Dyspepsia, diarrhea, nausea |
| Respiratory | Nasal congestion |
| Genitourinary | Priapism |
| Dermatological | Rash |
Sources: Goodman & Gilman's Pharmacological Basis of Therapeutics, PDE5 Inhibitors section; The Harriet Lane Handbook 23rd Ed., Sildenafil entry.
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