How Does Viagra Work? What Do I Do If It Doesn’t?
Written byAlex Brewer, PharmD
Medically Reviewed byAlex Brewer, PharmD
June 1, 2025 (Updated June 1, 2025)
Viagra®. The Blue Pill. The first oral medication approved by the Food and Drug Administration (FDA) for treating ED. Ever wondered how it works?
If your doctor prescribes Viagra, you’re likely curious as to what it does to your body. Understanding how a medication works helps you get the most out of treatment and minimizes your risk for side effects.
Viagra (sildenafil) has been used by millions to treat ED since its approval by the FDA in 1998. It’s a safe and effective treatment when used correctly at recommended doses. This guide discusses how Viagra works to treat ED, beginning with a discussion of what causes ED before going into detail on what Viagra does to your body and what to expect from treatment.
How does Viagra work to treat ED?
To understand how Viagra works to treat erectile dysfunction (ED), it’s vital to first understand what causes ED.
What is ED?
With erectile dysfunction (ED), you have trouble getting or maintaining an erection suitable for sex. ED affects different people differently. Some people only have trouble getting or maintaining an erection, while others will experience a mix of both symptoms. And your symptoms can change over time.
One key point: ED symptoms need to happen consistently over time for an official diagnosis. Having an occasional “off night” doesn’t mean you have ED. But if your symptoms persist over time, you should seek medical advice and learn about your treatment options.
In order to have and maintain an erection, numerous processes must go right in your body. There are both physical and psychological factors to getting and maintaining an erection.
When you’re aroused, signals sent from the brain cause increased blood flow into the penis, causing it to become hard and enlarged. But your blood vessels must be healthy enough to relax and allow blood flow into the penis.
Your nerves also need to be healthy. Otherwise, nerve signals that regulate blood flow into the penis may be interrupted or not sent at all.
ED can also be caused by psychological and mental health factors, even if your body is in healthy physical shape. Performance anxiety and depression can cause ED even though they don’t directly affect blood vessels or blood flow in the same manner as damaged blood vessels or nerves.
What’s in Viagra?
Viagra contains sildenafil as its active ingredient. An active ingredient is what makes a medication work. Sildenafil is a type of medication called a phosphodiesterase type 5 inhibitor (PDE-5 inhibitor). PDE5 inhibitors are considered first-choice treatment options for ED.
Sildenafil is also the generic version of Viagra. Like Viagra, sildenafil is FDA-approved to treat ED. It’s been found as safe and effective as Viagra. And it’s often much cheaper than the brand-name version.
Sildenafil uses
Viagra and sildenafil are used to treat erectile dysfunction (ED). This is the only approved use for brand-name viagra.
But sildenafil is also the active ingredient in the medication Revatio, which is used to treat high blood pressure in the lungs (pulmonary hypertension). Your healthcare provider likely won’t prescribe Viagra if you’ve had an allergic reaction to Revatio. And if you’re currently taking Revatio, your doctor likely won’t prescribe Viagra to you. Taking these medications together can cause dangerously low blood pressure.
What does Viagra do to your body?
Viagra contains sildenafil, which is a type of medication called a phosphodiesterase type 5 inhibitor (PDE-5 inhibitor).
PDE-5 is an enzyme found, among other areas, in smooth muscle cells in your blood vessels. PDE-5 acts to tighten smooth muscle in the penis, which prevents blood flow in and pushes blood out of the penis. When sildenafil blocks the PDE-5 enzyme, your blood vessels open and smooth muscle in the penis relaxes, allowing blood flow necessary for getting hard and maintaining an erection suitable for sex.
Viagra works the same as other FDA-approved PDE5 inhibitors such as Cialis® (tadalafil) and Levitra® (vardenafil).
What does Viagra do to your sexual experience?
Put simply, Viagra allows you to enjoy sex again.
If you have ED, worry over your ability to get or maintain an erection can cause performance anxiety. This can create a nasty negative spiral, where concern over your performance leads to worsening ED symptoms. With Viagra, you don’t need to worry about being hard enough for sex. You can instead focus on enjoying your partner and your experience.
A friendly reminder that Viagra does not improve sex drive and may not work if you’re not aroused. It’s not a treatment for low libido. If you have issues with your sex drive, let your healthcare professional know.
Does sildenafil work? Effectiveness of Viagra
Viagra (sildenafil) and other PDE-5 inhibitors are considered first-choice treatment options for ED. This is based on studies showing how well Viagra works and its safety when used as recommended.
For example, the American Urological Association (AUA) recommends PDE-5 inhibitors as first-choice treatment options in their guidelines for treating ED. The AUA is made up of healthcare professionals who specialize in urology, including expertise in men’s sexual health. They create these guidelines by looking at data from around the world to determine the best current treatment options for conditions like ED. These guidelines change over the years as new treatments or evidence are discovered. But PDE-5 inhibitors like Viagra have remained first-choice treatments for ED for many years, due to how well they work and their safety when used as directed.
Viagra clinical studies that led to its approval involved over 3,000 participants, with an age range of 19 to 87 years. Across 21 different studies, people taking Viagra reported a significant improvement in their ED symptoms and ability to have sex compared to people who used a placebo (pill with no active drug).
How long does it take for Viagra to start working?
It typically takes Viagra 30 to 60 minutes to start working. You can take Viagra anywhere between 30 minutes and 4 hours before sex.
Eating a high fat meal before or with your Viagra dose can make the medication take longer (usually around an hour or so) to begin working. Taking the medication on an empty stomach may help if Viagra seems to take a long time to work.
How long does Viagra last?
On average, Viagra makes its way through your system after a couple of hours. Exactly how long Viagra lasts can differ based on factors such as other medical conditions you have.
Viagra’s effects might last around four hours, meaning this is the window after taking a dose in which you may get an erection with sexual arousal and stimulation.
What should you do if Viagra doesn’t seem to work?
For some, Viagra works right away and continues working for a long time. But what should you do if Viagra doesn’t seem to work?
Incorrect use
The most common reason Viagra and other PDE-5 inhibitors don’t work is due to incorrect use, according to studies from the American Urological Association.
How does one incorrectly use Viagra? Interestingly, a frequent cause is lack of sexual stimulation and sexual arousal. Contrary to popular perception, Viagra doesn’t “force” you to become hard. As noted above, Viagra doesn’t impact your libido or sexual desire. In order for Viagra to work, you need sexual arousal and stimulation.
Incorrect use also includes not following directions from your healthcare provider. Always use Viagra exactly and only as directed by your doctor. Never change your dose of Viagra on your own. Speaking off.
Ask your doctor: Adjusting Viagra dosage
If Viagra doesn’t seem to work, talk with your healthcare provider about adjusting your Viagra dosage. The recommended starting dose of Viagra is 50 mg, but the maximum dose is 100 mg. If the 50 mg dose does not work for you, your healthcare provider may increase your dose of Viagra to 100 mg.
Ask your doctor: Switching PDE5 inhibitors
You’ve tried different Viagra doses and confirmed with your healthcare professional you’re using it correctly, but Viagra still isn’t working. What now?
Your doctor may suggest stopping Viagra and trying a different PDE5 inhibitor. Examples include Cialis (tadalafil) and Levitra (vardenafil). Switching treatment options and trying different PDE5 inhibitors can work, but you shouldn’t do this on your own without approval from your doctor.
Viagra side effects
Most medications can cause side effects, and Viagra is no exception. However, Viagra is actually a relatively safe treatment when used correctly as recommended by your healthcare professional. It’s been safely used by millions over several decades since it became FDA-approved.
Common side effects
The most common side effects reported by people taking Viagra in clinical trials are:
- Headache
- Flushing
- Indigestion
- Stuffy nose
- Temporary vision changes, including blurry vision or changes to color vision
Common side effects of Viagra are usually temporary and go away within a few hours or days. Let your healthcare professional know if you have side effects that last or interfere with daily activities.
Mild side effects
Mild side effects of Viagra include:
- Headache
- Flushing
- Indigestion
- Runny or stuffy nose
- Temporary vision changes, including blurry vision or changes to color vision
- Back pain
- Muscle pain
- Nausea
- Dizziness
- Rash
Serious side effects
Serious side effects of Viagra include:
- Allergic reaction
- Hearing changes, including temporary decreased hearing or hearing loss
- Heart-related side effects, which are unlikely unless you have an existing heart condition (such as heart disease or a past heart attack)
- Eye and vision-related side effects, including permanent vision loss
- Priapism (a prolonged, often painful erection lasting four hours or longer)
- Allergic reaction
Talk with your healthcare professional if you have questions about Viagra side effects.
Frequently asked questions
Maybe. Viagra is a type of medication called a PDE5 inhibitor. And studies have shown PDE5 inhibitors may treat premature ejaculation in people with ED. There are also new studies looking at using sildenafil as a gel to treat premature ejaculation. But more research is needed before we know how well this works.
If you don’t have ED, it’s less clear whether PDE5 inhibitors work for premature ejaculation. But you can ask your healthcare professional to learn about other treatment options for premature ejaculation, such as behavioral techniques, counseling, and certain medications. Some antidepressants are used first-choice to treat premature ejaculation, but they can also contribute to ED. So it’s important to review possible treatment options with your doctor to determine the best for you.
No. Viagra will not increase your penis size. It’s not unusual to worry about your penis size. But it’s important to note this is often a perception problem. Most penises are sizeable enough for sexual activity, and the worry over whether your size is “normal” or “enough” can further worsen erectile dysfunction.
Talk with your doctor if you’re concerned or worry about your penis size. They might recommend talking with a mental health expert, which can help with your anxiety - and this can help you get the most out of Viagra.
After you take Viagra, the drug begins working to relax blood vessels and smooth muscle in the penis. This increases blood flow, which is necessary to get and maintain an erection suitable for sexual activity. Viagra also makes it harder for blood to flow out of the penis prior to ejaculation, which helps you stay hard enough for sexual activity.
It depends. Original studies of Viagra didn’t evaluate how long erections lasted. Researchers mainly wanted to know whether people who took Viagra were able to get an erection suitable for sex, not how long the erection lasted.
One study that involved 28 men taking Viagra reported an erection lasted an average of 31 minutes. Another study found men with spinal cord injuries who took Viagra had an average erection last about 34 minutes. But keep in mind your results may vary. Generally, Viagra should enable you to get and maintain an erection that is hard enough for sex.
It’s important to know how long an erection shouldn’t last with Viagra. An erection lasting four hours or longer needs immediate medical attention, as this is likely a medical emergency called priapism - a prolonged and often painful erection. Priapism must be treated as soon as possible, or it can cause permanent damage to the penis.
Yes, Viagra does have drug interactions. Certain drug interactions can make Viagra not work or not work as well. Other interactions can increase your risk for side effects of Viagra.
Nitrates, such as nitroglycerin, and nitrites or recreational “poppers”, such as amyl nitrite, should not be taken with Viagra. Doing so can cause a dangerous drop in blood pressure. You also shouldn’t take other PDE5 inhibitors, such as Cialis (tadalafil) and Levitra (vardenafil), with Viagra. Doing so raises your risk for side effects. This also applies to PDE5 inhibitors that treat pulmonary arterial hypertension, such as Revatio (sildenafil) and Adcirca (tadalafil). And avoid taking GC stimulators, such as riociguat (Adempas), if you’re taking Viagra.
Other interactions can raise your risk for side effects as well. But the risk is lower, and the combination may be OK if your doctor says so. For example, alpha blockers, such as doxazosin and tamsulosin, can cause a drop in blood pressure if used with Viagra. But the risk isn’t as high as with nitrates. The same goes for antihypertensives, such as lisinopril, valsartan, and carvedilol. For these interactions, your doctor may lower the dose of the antihypertensive or alpha blocker. Or they may prescribe a lower Viagra dose.
Some medications affect how your body breaks down (metabolizes) Viagra. Examples of these medications include HIV protease inhibitors, such as ritonavir (Norvir) or atazanavir (Reyataz), certain antifungal drugs, including ketoconazole and itraconazole, and certain antibiotics, including erythromycin and clarithromycin. For these interactions, your doctor will likely prescribe a lower Viagra dose to lower your risk for Viagra side effects.
To help avoid interactions, tell your doctor and pharmacist about all prescription drugs and supplements you take, including any over-the-counter medications.
Certain people should not Viagra due to the risk for serious side effects. If you take a guanylate cyclase (GC) stimulator, such as riociguat (Adempas), your doctor likely won’t prescribe Viagra. They also likely won’t prescribe Viagra if you’re allergic to sildenafil or any other ingredient. If you’re prescribed Viagra, do not use amyl nitrate, amyl nitrite, butyl nitrate, or other “poppers”. Doing so raises your risk for low blood pressure as a side effect.
Some people need to use Viagra with caution, such as if you have Peyronie’s disease or another anatomical problem with your penis. If you have a medical condition that raises your risk for priapism (a known Viagra side effect), such as sickle cell anemia, your doctor should discuss the risks and benefits of taking Viagra to decide if treatment is safe for you. If you have heart problems, especially if you’ve had a heart attack or heart surgery in the past 6 months, do not take Viagra without discussing your heart health with your doctor.
Finally, let your doctor know if you have a condition affecting your liver or kidney. Kidney and liver conditions can impact how well your body gets Viagra out of your system, which can raise your risk for side effects.
No, not exactly. Viagra is one of several treatment options for ED, but it’s not a cure. If you stop using Viagra, your ED symptoms will likely return.
Key Takeaways
- Viagra is a safe and effective FDA-approved treatment option for erectile dysfunction (ED)
- Both Viagra and sildenafil (generic Viagra) work the same. They’re both FDA-approved, and sildenafil is equally effective and safe. Generic Viagra is typically much more affordable.
- Viagra works by improving and maintaining blood flow to the penis, which is required for an erection suitable for sex
- Sexual stimulation is required for Viagra to work.
- If Viagra doesn’t work for you, talk with your doctor. Your doctor may increase your Viagra dosage to see if this helps. You should never change your dose of Viagra on your own.
This article offers general information and is not intended as medical advice. It does not replace the expertise of a licensed healthcare professional, who should be consulted regarding any medical questions or treatment decisions. Always seek personalized guidance from your doctor before starting or changing any treatment. While efforts are made to ensure accuracy, the content may not address all potential risks, possible uses or benefits.
