Sildenafil: what it treats, how to take it, side effects
Last updated September 3, 2026.
Sildenafil is a PDE5 inhibitor - it increases blood flow to the penis during sexual stimulation. You may know it as Viagra for erectile dysfunction; under the names Revatio and Liqrev, the same drug treats pulmonary arterial hypertension. The one rule that overrides everything: never combine it with nitrates.
What does it treat?
Erectile dysfunction in men, and pulmonary arterial hypertension - high blood pressure in the vessels carrying blood to the lungs - in adults and, as Revatio, children 1 year and older. For ED, it does not cure the condition or increase sexual desire, and it does not prevent pregnancy or sexually transmitted diseases. Only one sildenafil product at a time, ever.
How do you take it?
For erectile dysfunction: as needed, before sexual activity - about 1 hour before is the sweet spot, but anywhere from 30 minutes to 4 hours ahead works. Never more than once in 24 hours. With or without food, but a high-fat meal slows it down. The label's usual starting dose is 50 mg, adjusted between 25 mg and 100 mg based on response; if you take an alpha blocker, the label says start at 25 mg only after your alpha-blocker dose is stable. For PAH: three times a day at even intervals, same times every day. If it is not working well or side effects are bothering you, tell your doctor - do not adjust on your own.
Side effects to know
- Common: headache, heartburn, diarrhea, nausea, flushing, nosebleeds, numbness or tingling in the arms, hands, feet, or legs, muscle, back, arm, or leg pain, color-vision changes like a blue tinge or trouble telling blue from green, light sensitivity, and nasal congestion.
- Serious - call your doctor immediately: sudden severe loss of vision, blurred vision, sudden decrease or loss of hearing, ringing in the ears, dizziness or fainting, chest pain, worsening shortness of breath, an erection that is painful or lasts longer than 4 hours, and allergic reactions with rash, hives, trouble breathing or swallowing, or swelling of the eyes, face, throat, tongue, or lips.
Who should not take it?
Anyone on nitrates for chest pain - nitroglycerin, isosorbide - or using nitrate street drugs ("poppers"): the combination craters blood pressure, and MedlinePlus says your doctor will tell you not to take sildenafil. Flag heart attack, stroke, heart failure, angina, irregular heartbeat, high or low blood pressure, heart surgery in the last 6 months, bleeding disorders, sickle cell anemia, multiple myeloma, leukemia, conditions affecting the shape of the penis, a past erection lasting over 4 hours, stomach ulcer, kidney or liver disease, retinitis pigmentosa, and any past sudden vision loss. Cimetidine and St. John's wort interact. Grapefruit is a doctor question. And if sexual activity itself has ever been flagged as a strain on your heart, that conversation comes first - chest pain, dizziness, or nausea during sex means stopping and calling your doctor.
When is it an emergency?
An erection lasting longer than 4 hours is immediate care - waiting causes permanent damage. Sudden loss of vision or hearing means calling immediately and taking no further doses until you do. Chest pain during sex, or trouble breathing, is a 911 call - and tell the responders when you last took sildenafil, because it changes what they can give you. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why can't I take sildenafil with nitrates?
Both drop blood pressure, and together they drop it dangerously. MedlinePlus is absolute: if you take nitrates for chest pain - nitroglycerin, isosorbide - your doctor will tell you not to take sildenafil, and the same goes for nitrate street drugs like poppers. This is also why emergency staff need to know when you last took it: it changes what heart medications they can use.
How far in advance should I take sildenafil?
About an hour before sexual activity is the standard, but the window runs from 30 minutes to 4 hours. Two practical notes from MedlinePlus: a high-fat meal slows the onset, and it needs sexual stimulation to work - it increases blood flow in response to arousal, it does not create arousal.
Why do I see a blue tinge after taking sildenafil?
A known, usually harmless effect. MedlinePlus lists color-vision changes - a blue tinge on objects or trouble telling blue from green - and light sensitivity among the common effects, from the drug touching a related enzyme in the eye. It passes as the drug wears off. The line not to cross: sudden vision loss, with or without these changes, means calling your doctor immediately and taking no more until you do.
Can I drink alcohol with sildenafil?
Neither the label nor MedlinePlus gives sildenafil-specific alcohol language. Alcohol lowers blood pressure the same direction the drug does, and it works against the erection you are taking it for. Light or none is the practical call - get the personal rule from your doctor.
Is it safe to buy sildenafil online without a prescription?
The risk is the counterfeits, and the risk you cannot see: the nitrates rule. Sildenafil has a genuine dangerous-combination list - nitrates, certain other drugs, starting doses that change with alpha blockers - and the prescriber visit exists to check yours. A pill of unknown contents with none of that screening is how the emergency-room stories happen.
What is the difference between Viagra and Revatio?
Same drug, different job and schedule. Viagra is sildenafil for erectile dysfunction, taken as needed. Revatio (and Liqrev) is sildenafil for pulmonary arterial hypertension, taken three times a day on a fixed schedule. MedlinePlus warns you should only ever be treated with one sildenafil product at a time.