Side Effects of Viagra (Sildenafil)
The most common side effects of Viagra are headache, flushing, dyspepsia, and nasal congestion, which occur due to its vasodilatory mechanism and are typically mild to moderate in severity. 1, 2
Common Vasodilatory Side Effects
The majority of adverse effects are directly related to sildenafil's mechanism of smooth muscle relaxation and vasodilation:
- Headache is the most frequently reported side effect across all clinical trials 2
- Flushing (facial redness and warmth) occurs commonly due to peripheral vasodilation 1, 2
- Dyspepsia (upset stomach/epigastric discomfort) results from relaxation of the gastroesophageal sphincter with reflux 1, 2, 3
- Nasal congestion and epistaxis (nosebleeds) are common vasodilatory effects 1, 2
- Rhinitis occurs frequently 1
Less Common Side Effects
Additional adverse effects reported in clinical trials include:
- Visual disturbances, including altered color perception (due to phosphodiesterase-6 inhibition) 1
- Myalgia (muscle pain) 1
- Nausea, diarrhea, and vomiting 1
- Dizziness 1
- Chest pain 1
Serious Adverse Effects (Rare)
Priapism (erections lasting more than 4 hours) is rare but requires immediate medical attention, though it was reported infrequently during postmarketing surveillance rather than in clinical trials 1, 2, 4
Visual complications: Very-low-quality evidence from 4 million veterans showed PDE-5 inhibitors were not associated with increased risk for nonarteritic anterior ischemic optic neuropathy (NAION), but were associated with increased risk for "possible NAION" (absolute risk 2.4 cases per 10,000 men per year) 1, 2
Cardiovascular events (heart attack, stroke, irregular heartbeats, and death) have been reported, though most occurred in men who already had underlying heart problems 4
Overall Safety Profile
- Most side effects are transient and mild to moderate in severity 2, 5, 3
- The overall discontinuation rate due to adverse effects in clinical trials is approximately 3% 2
- Tolerability is maintained in long-term studies up to 1 year 5
- No serious sildenafil-related adverse events occurred in controlled clinical trials; cardiovascular events in postmarketing surveillance generally occurred in patients with pre-existing risk factors 5
Critical Safety Warnings
Absolute contraindication with nitrates: Concurrent use of sildenafil with any nitrate medication (nitroglycerin, isosorbide dinitrate) can cause severe, potentially fatal hypotension due to synergistic vasodilation 1, 2, 4, 6, 3
Drug interactions: Medications that inhibit cytochrome P-450 (cimetidine, ketoconazole, erythromycin, ritonavir) can increase plasma sildenafil concentrations, though this does not typically translate into clinically significant effects 4, 6, 3
Clinical Pearls
- The incidence of serious adverse events is less than 2%, with no difference between sildenafil and placebo 1
- Side effects are dose-related, so titrating to the lowest effective dose can minimize adverse effects 1
- If a patient on sildenafil requires emergency nitrate administration, wait at least 24 hours after the last sildenafil dose before giving nitrates, under close medical supervision 1, 7