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How does Sildenafil (Viagra) work?
Sildenafil (Viagra) is a phosphodiesterase-5 (PDE-5) inhibitor. PDE-5 is a protein in your body that breaks down the molecules that normally cause an erection. When it blocks PDE-5, sildenafil (Viagra) helps an erection occur by allowing certain muscles in the penis to relax and the penis to fill with blood.
Sildenafil works in different ways, depending on the condition it’s being used to treat.

For ED: Sildenafil works by increasing blood flow to the penis. This helps you get or maintain an erection. Sildenafil only works for ED if you’re sexually aroused.
For PAH: Sildenafil works by relaxing the muscles and opening up the blood vessels in your lungs. This decreases the blood pressure in your lungs. Sildenafil slows down the progression of your illness. It also makes you more able to exercise.

How to take sildenafil
All possible dosages and drug forms may not be included here. Your dosage, drug form, and how often you take the drug will depend on:

your age
the condition being treated
how severe your condition is
other medical conditions you may have
how you react to the first dose

What’s the difference between sildenafil and Viagra?
The main difference between generic sildenafil and Viagra is marketing. The main ingredient in Viagra is sildenafil, meaning that the drugs are essentially the same.
However, Viagra is marketed and prescribed strictly for erectile dysfunction, while generic sildenafil can be prescribed for PAH as well.
Viagra is also known as “the little blue pill” because of its blue color, while generic sildenafil tends to be a white color.

What are the side effects of Sildenafil (Viagra)?
Contact your healthcare provider immediately if you experience any of the following.

Erection lasting for more than 4 hours
Vision loss
Dangerously low blood pressure: feeling faint or weak, dizziness, blurry vision, feeling confused

Common Side Effects

Changes in vision or light sensitivity
Stuffy nose
Back pain
Muscle ache


Store sildenafil tablets at room temperature between 59°F and 86°F (15°C and 30°C).
Keep sildenafil tablets away from light.
Don’t store this medication in moist or damp areas, such as bathrooms.

1. How long does it take for sildenafil (Viagra) to work?
Typically, people notice the effects of sildenafil (Viagra) in about a hour after taking the medication. But you can take it anywhere from 30 minutes to 4 hours before sexual activity. Keep in mind that if you happen to take it after a high fat meal (e.g., cheeseburger and fries), it might take the medication a little longer to kick in.

2. Can I take sildenafil (Viagra) multiple times a day?
Don’t take more than one dose of sildenafil (Viagra) per day, even if you have sexual activity more than once. Taking more than the recommended dose of sildenafil (Viagra) puts you at risk for serious side effects, such as heart problems, vision problems, or permanent damage to the penis. If you have questions about your dose of sildenafil (Viagra), talk with your provider.

3. Should I take sildenafil (Viagra) if I don’t have erectile dysfunction (ED)?
Sildenafil (Viagra) is only FDA-approved for people with ED. If you haven’t been diagnosed with ED by your healthcare provider, don’t take sildenafil (Viagra) for sexual performance. This medication can put you at risk for serious side effects, such as dangerously low blood pressure, vision loss, and damage to the penis. If you think you might have ED and want to know if sildenafil (Viagra) is right for you, talk with your healthcare provider.