Quick facts
Generic name: sildenafil citrate · Class: PDE5 inhibitor · FDA approved: 1998 · Onset: ~30–60 min · Duration: ~4–6 hours · Status: prescription only
Background & discovery
Sildenafil was originally investigated as a treatment for angina (chest pain). During trials, researchers noticed a striking side effect — improved erections — and the drug was redirected toward erectile dysfunction. In 1998 it became the first oral ED therapy approved by the U.S. Food and Drug Administration, marketed as Viagra, and it transformed how ED is treated.
How sildenafil works
Sildenafil blocks the enzyme phosphodiesterase type 5 (PDE5). By doing so it preserves cyclic GMP, the molecule that relaxes the smooth muscle of blood vessels in the penis and increases blood flow during arousal. The result is an easier, firmer erection — but only in response to sexual stimulation. The medication enhances the natural response rather than creating an erection by itself.
Onset, duration & how to take it
Sildenafil is typically taken about 30 to 60 minutes before sexual activity, and its effect lasts roughly 4 to 6 hours. A high-fat meal can slow absorption and delay the onset, so many men take it on a relatively empty stomach. It is intended for use as needed rather than on a daily schedule, and should not be taken more than once in a 24-hour period.
What clinical research shows
Sildenafil has been evaluated in a large number of randomized, placebo-controlled trials across men with ED of many causes — including diabetes, cardiovascular disease and post-prostatectomy ED. In these studies a substantial majority of men reported improved erections and more successful intercourse compared with placebo. Decades of post-marketing experience have reinforced its effectiveness and characterized its safety profile in detail.
Beyond ED, the same molecule (under the brand Revatio) is approved at different doses to treat pulmonary arterial hypertension — a reminder that these are powerful cardiovascular medications that deserve medical supervision.
Common side effects
- Headache
- Facial flushing
- Nasal congestion
- Indigestion
- Temporary blue-tinted or light-sensitive vision (dose-related, usually mild)
Most side effects are mild and short-lived. Rare but serious effects can occur, including priapism (a prolonged, painful erection) and sudden changes in vision or hearing, which require immediate medical attention.
Important safety information
- Do not use with nitrates. Combining sildenafil with nitrate heart medications or "poppers" can cause a life-threatening drop in blood pressure.
- Use caution with alpha-blockers and other blood-pressure medications.
- Discuss your heart health. Men with significant cardiovascular disease should be evaluated before use.
- Only obtain it with a prescription from a licensed pharmacy. Counterfeit "Viagra" sold online is widespread and may contain unknown or harmful ingredients.
Frequently asked questions
How long does Viagra take to work?
Usually 30 to 60 minutes. Taking it on an empty stomach can help it work faster, since a high-fat meal may delay absorption.
How long does the effect last?
About 4 to 6 hours. It does not cause a continuous erection — sexual stimulation is still needed for it to work.
Can I drink alcohol with it?
Moderate alcohol is generally not dangerous, but heavy drinking can worsen ED and increase side effects like dizziness. Ask your physician about your situation.
Is generic sildenafil the same as Viagra?
Generic sildenafil contains the same active ingredient and is held to the same FDA standards. Your physician or pharmacist can advise on dosing and cost.
Related reading: Generic Viagra (sildenafil): is it safe & how to check quality · Viagra vs Cialis vs Levitra · Compare all ED medications