How does Viagra (sildenafil) work?

You’ve probably heard of Viagra (sildenafil), also known as the “little blue pill.” But how much do you know about how it works? 

Contrary to popular belief, Viagra doesn’t simply give you an erection—rather, it helps you get one (and keep it) when you’re in the appropriate mood and need it most.  

Viagra works by blocking what’s called PDE5 (phosphodiesterase type 5), an enzyme in the body that breaks down chemicals responsible for erections. When PDE5 is blocked, these chemicals don’t get broken down, and as a result, erectile tissues in the penis stay relaxed and engorged with blood. If you’re unsure how that relates to an erection, we got you. 

Erections seem pretty basic, but they're surprisingly complicated. Getting an erection involves cooperation between your heart, blood vessels, hormones, nerves, and even your mood. 

During an erection, chemicals in the body cause the smooth muscle in the penis to relax. As a result, blood flowing into the penis gets trapped (you’ll notice much more blood in an erect penis than when it’s flaccid). Eventually, the blood gets released, the erection subsides, and blood flow down there returns to normal. 

When you have erectile dysfunction (ED), this process doesn’t work as reliably as it should. A variety of things can cause ED. Age is a big factor, as the risk of ED increases as you move past 40. Other medications can also contribute to ED, as can underlying health conditions like high blood pressure, diabetes, or depression. That’s why it’s a good idea to make an appointment with your healthcare provider once you notice a change in your erections. Not only can they write you a prescription for Viagra (if appropriate), but they can also help you understand what’s causing your ED—and recommend treatment.

Does Viagra make you hornier? No. While Viagra is highly effective for ED, it is not a magic pill: you need to feel sexually aroused for it to work. Then, Viagra helps you get and maintain an erection.

For most men, Viagra starts working within 30–45 minutes. Although, it can take longer for Viagra to work for some men. For best results, the makers of Viagra recommend taking it around 1 hour before you plan on having sex. And for a lucky few, Viagra kicks in even faster—within just 12 minutes, according to one study. 

To ensure Viagra starts working as quickly as possible, avoid taking Viagra with a high-fat meal, such as a cheeseburger and fries. This can delay the onset of Viagra by up to 60 minutes.

Viagra is effective for at least 4 hours after you take it. However, the effects typically peak between 30 minutes and 2 hours after you take Viagra. That’s why the makers of Viagra recommend taking it around 1 hour before sexual activity. But, as long as you have sex within 4 hours of taking Viagra, the medication should work. 

How long Viagra lasts in your system can depend on several factors, including your age, the dosage, other medications you are taking, and your overall health. Even what you eat can have an effect, as taking Viagra with fatty foods can delay how long it takes to kick in.

Your healthcare provider will tell you how to take Viagra, including how much to take and when.  Generally, however, you only need to take Viagra as needed, around 1 hour before you plan on having sex. You don’t need to take it daily (unless you plan on having sex daily), and you should never take it more than once per day. 

You can take Viagra with or without food. But, if you do take it with food, avoid taking Viagra with a high-fat meal, as that can make it take longer to start working. 

Common side effects of Viagra and other ED medications include:

• Headache

• Facial flushing

• Nasal congestion

• Upset stomach (indigestion)

• Temporary vision changes (e.g. hazy vision, changes in color or light perception),

• Muscle or back pain

• Dizziness

• Skin rash

Other less common side effects include chest pain, shortness of breath, low blood pressure, and fainting. 

A well-known but rare reaction to Viagra is priapism, a painful erection that won't go away. If an erection lasts more than a few hours, it can damage the penis (that's why ED medications have warnings about erections "lasting longer than four hours"). Other rare but potentially serious side effects of Viagra include sudden vision or hearing loss. If you experience any of these serious side effects, seek medical attention ASAP. 

Viagra by itself can also lower blood pressure. If you already have low blood pressure or are taking medication to treat high blood pressure, talk with a healthcare provider before taking Viagra. 

Treatment with PDE5 inhibitors is not safe for everyone. For example, people taking certain drugs for heart disease (like nitrates) should not use Viagra. 

Nitrates increase what’s called nitric oxide, which triggers the release of erection-inducing chemicals. Problems arise if you take ED meds with nitrates––like nitroglycerin for chest pain or amyl nitrite (poppers). The combination can cause too much blood vessel relaxation (vasodilation). This can potentially lead to a dangerous, even fatal, drop in blood pressure.

People with a history of heart problems or kidney or liver issues should check with a medical professional before taking Viagra. If you’ve experienced allergies or a hypersensitivity reaction to Viagra, you shouldn’t use the medication again.

It’s also important to tell your healthcare provider about any other medications you take, as some may not be safe to take with Viagra. Drug interactions include guanylate cyclase (GC) stimulators, alpha blockers, antifungals, and medications that treat high blood pressure. 

If your healthcare provider determines Viagra isn’t safe for you, they may recommend another ED medication. Viagra is one of several PDE5 medications that treat ED. Other medications include Cialis (tadalafil), vardenafil (formerly known as Levitra and Staxyn), and Stendra (avanafil). There are also ED gummies and combination treatments like Ro Sparks.

Whether or not Viagra is right for you, it’s helpful to remember that medication isn’t the only answer for ED. Certain lifestyle changes can also help, including exercise, a better diet, and managing stress. You may feel embarrassed to talk about ED, but don’t be. ED affects millions of men, and you owe it to yourself to speak with an expert about your concerns. Get the medical advice you need to take back control of your health.

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Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

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Current version

September 23, 2024

Written by

Michael Martin

Fact checked by

Patricia Weiser, PharmD

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About the medical reviewer

Patricia Weiser, PharmD

Patricia Weiser, PharmD, is a licensed pharmacist, health content writer, and medical reviewer with more than a decade of clinical experience in community and hospital pharmacy.