Sildenafil (Viagra) Side Effects: Common and Serious Risks
Like any medication, sildenafil carries a known side effect profile, ranging from mild and common to rare but serious. This page summarizes what has been reported in clinical trials and post-marketing surveillance, the specific drug interactions that matter most, and who should avoid sildenafil altogether.
Common Side Effects
Most people who experience side effects from sildenafil describe them as mild and temporary. The most frequently reported include:
- Headache
- Facial flushing
- Nasal congestion or stuffy nose
- Indigestion or upset stomach
- Temporary visual changes, including a mild blue tinge to vision or increased light sensitivity
- Dizziness
These effects are generally dose-related, meaning they occur somewhat more often at 100 mg than at 25 mg, and typically resolve on their own within a few hours as the drug clears the body.
Key point: Mild headache, flushing, and congestion are the most commonly reported sildenafil side effects and usually resolve within a few hours.
Serious Side Effects
Serious side effects are uncommon but important to recognize. These include a sudden decrease or loss of vision in one or both eyes (associated with a rare condition called NAION), sudden hearing loss, an erection lasting longer than four hours (priapism), and chest pain during sexual activity. Any of these requires prompt medical attention — an erection lasting more than four hours is considered a medical emergency because it can permanently damage erectile tissue if not treated quickly.
Important: Sudden vision loss, sudden hearing loss, or an erection lasting longer than four hours are medical emergencies. Seek immediate care if any of these occur.
The Nitrate Interaction
The most important interaction to know about is with nitrate medications, commonly prescribed for chest pain (angina), such as nitroglycerin. Nitrates and sildenafil both lower blood pressure through related pathways, and combining them can cause a severe, sudden, and potentially life-threatening drop in blood pressure. This is why prescribers ask directly about nitrate use before writing a sildenafil prescription, and why sildenafil should never be taken by anyone currently using any form of nitrate, including short-acting versions used only occasionally.
Other Interactions Worth Knowing
Alpha-blocker medications, often prescribed for high blood pressure or an enlarged prostate, can have an additive blood-pressure-lowering effect when combined with sildenafil, so dose timing or spacing is sometimes adjusted. Certain medications that inhibit the liver enzyme CYP3A4, including some antifungal and antibiotic drugs, can raise sildenafil levels in the blood, which may prompt a lower starting dose. Grapefruit and grapefruit juice can have a similar effect and are often best avoided around dosing time.
Who Should Avoid Sildenafil
- Anyone taking nitrate medications in any form
- Anyone with a recent heart attack, recent stroke, or unstable angina
- Anyone with uncontrolled low blood pressure
- Anyone with certain inherited eye conditions, such as retinitis pigmentosa, without specific prescriber guidance
- Anyone with severe liver impairment, without dose adjustment guidance from a prescriber
Sildenafil vs. Tadalafil Side Effect Patterns
While the two drugs share the same core warnings, some side effects appear at slightly different rates. Sildenafil is somewhat more associated with visual disturbances, while tadalafil is somewhat more associated with back pain and muscle aches. Neither pattern is universal, and individual experience varies. See our tadalafil side effects page for the comparison, or our full Viagra vs Cialis comparison for a side-by-side look at both drugs.