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Is Sildenafil Safe for Erectile Dysfunction (ED)?

Mike Bohl, MD, MPH, ALM

Reviewed by Mike Bohl, MD

Written by Lauren Panoff

Published 09/06/2021

Updated 01/06/2024

In the ‘80s, researchers set out to test a medication called sildenafil citrate for treating high blood pressure and angina (chest pain) from heart disease. While the treatment appeared promising for these purposes, it proved to be more potent in — ahem — another more sensitive area of the body. 

Many of the male participants were embarrassed to disclose to the research staff that they were experiencing erections during the trial.

While sildenafil was intended to relax and dilate blood vessels around the heart, it was also active in the blood vessels around the penis. That meant it increased blood flow around the heart and also to the genitals, hence the erections. Talk about getting more than you bargained for.

Needless to say, researchers took an unexpected pivot and decided to investigate sildenafil as a treatment for erectile dysfunction (ED). After all, if it was causing erections among men who weren’t necessarily looking for that effect, imagine what it could do for men who were struggling to stay hard.

After further trials, sildenafil citrate was approved by the Food and Drug Administration (FDA) in 1998 as the active ingredient in brand name Viagra®, now a very well-known drug for ED.

For most men, sildenafil is a safe and effective treatment for ED when used as prescribed. However, like all drugs, it comes with its own set of potential side effects and interactions. 

We’ve broken down how sildenafil works for treating ED, who shouldn’t use it and how to ensure that you’re using it safely to improve your sex life.

Thanks to the unintended discovery during those heart disease trials, we now have a good understanding of all the ways sildenafil works in the body.

Treating ED is the most prevalent and well-understood of uses for Viagra  — it’s also used in different dosages for treating pulmonary arterial hypertension under the brand name Revatio®. So, how does it work anyway? 

Sildenafil falls into a class of drugs called PDE5 inhibitors, along with other drugs like Cialis® (tadalafil), Stendra® (avanafil) and Levitra® (vardenafil). These are the gold standard drugs for the treatment of erectile dysfunction. In fact, studies show that just one dose of Viagra has a 70 to 80 percent success rate for treating ED. 

PDE5 stands for phosphodiesterase type 5, an enzyme that plays an important role in regulating blood flow to many parts of your body, including the soft erectile tissues inside your penis. 

Specifically, PDE5 can keep the smooth muscle in the arteries that supply your penis from relaxing, which can reduce blood flow. PDE5 inhibitors increase blood flow to your penis, resulting in longer, harder erections when you’re sexually aroused.

Your next question is probably when it will start working and how long sildenafil lasts. And we get it. This is important stuff when you’re planning the logistics of your evening. 

Most of the time, Viagra begins to work 30 to 60 minutes after taking it. 

It may work more slowly if you take it after eating a fatty meal. So make sure that you strategically pop that little blue pill based on what your plans are.

As for how long it lasts, the packaging insert says Viagra has a half-life of four hours.

No, this doesn’t mean you’re still going to have a hard-on at that point. It just means that the drug will still be in your system, but not necessarily at a high enough concentration to have noticeable effects. For the best results, plan to have sex within these first four hours of taking sildenafil.

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When used as directed to treat erectile dysfunction, sildenafil is safe and effective for most men. However, like any drug, it has potential side effects and drug interactions that are important to consider before using. 

The most common sildenafil side effects include: 

  • Headaches, which affect up to 28 percent of men who use it

  • Muscle aches and back pain

  • Stuffy nose, due to its dilating effects on blood vessels

  • Lightheadedness or dizziness

  • Flushing of the skin on your cheeks

  • Heartburn or indigestion, because of how sildenafil acts on the smooth muscle cells throughout your body

  • Nausea, though this is only reported by a small percentage of men who use it

  • Reduced blood pressure when combined with certain other medications

  • Blurred vision or otherwise abnormal vision, though this is usually mild and temporary

If you take a dose of sildenafil above 100mg, you’ll have a higher risk of experiencing any of these side effects.

Other more serious but less common side effects of Viagra have also been reported, such as:

  • Priapism, or a prolonged erection that lasts several hours and can become painful

  • Sudden hearing loss

  • Rare permanent loss of vision due to optic nerve damage

If you experience any of these potentially serious side effects, seek immediate medical attention. Your healthcare provider will be able to help you figure out whether they are likely to be temporary or more serious. If the side effects are serious or even if they just impact your life too much, they may recommend an alternative drug or another ED treatment. 

Sildenafil acts on the blood vessels in your penis, so you may wonder whether it has any worrisome effects on your cardiovascular system as a whole.

There have been reports of stroke, heart attack and irregular heartbeats among some people who take Viagra. However, there is currently no solid evidence that directly links sildenafil or other ED medications to heart attacks in men. 

The fact is that people who have ED are at a higher risk of having heart disease — and vice versa. For example, problems with both your penis and your heart can be associated with a vascular condition that affects blood flow. 

PDE5 inhibitors like sildenafil may not be safe for people who have existing heart problems. It's unclear if it is safe in among people who have had a heart attack, stroke or irregular heartbeat within the last six months, and you should talk to your doctor if you have abnormal blood pressure at rest or a history of heart failure or unstable angina.

A heart attack occurs when the supply of blood to the heart is abruptly blocked. It’s usually caused by coronary artery disease — a form of heart disease that involves the buildup of plaque inside the walls of the arteries that supply blood to your heart.

As a vasodilator, sildenafil actually increases blood flow to parts of your body, including to areas other than your penis. By doing so, it may lower blood pressure. This drop is generally so small that most men don’t notice it, even ones who are taking medication for high blood pressure.

Still, it can be dangerous — even life-threatening — if you have existing heart-related concerns. 

Additionally, sildenafil can interact with drugs called nitrates, which are commonly prescribed for heart-related conditions. We’ll discuss this more next. 

Finally, sexual activity of any type can put a strain on your heart. If you have an existing heart condition, the physical stress of having sex may increase your risk of heart attack, stroke or other cardiovascular events.

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There are certain types of medications that shouldn’t be taken with sildenafil due to potentially serious interactions, like:

  • Nitrates (e.g., nitroglycerin, isosorbide dinitrate or isosorbide mononitrate) used to treat congestive heart failure and  chest pain

  • Alpha-blockers, which relax blood vessels and are prescribed for hypertension and benign prostatic hyperplasia (BPH), a common prostate condition among aging men that can lead to enlarged prostate and urinary symptoms

  • Recreational drugs called “poppers,” which may contain nitrites

  • Ketoconazole, a drug used to treat certain fungal infections

When nitrate-containing drugs, poppers or alpha-blockers are used with sildenafil, they can cause a sudden and severe drop in your blood pressure. This may make you feel dizzy or, more seriously, could cause you to pass out.

Ketoconazole can inhibit certain proteins in your liver. This can result in higher-than-normal levels of sildenafil in your body, which may lead to headaches, flushing and dizziness.

Sildenafil can even potentially trigger a heart attack or stroke when combined with certain other medications. 

The best way to avoid adverse effects is to disclose all of your prescription, over-the-counter and recreational drug use to your healthcare provider before you begin using Viagra. 

Additionally, certain pre-existing medical conditions may increase your risk of experiencing side effects from using sildenafil. 

For instance, inform your healthcare professional if you have: 

  • A deformed penis shape or condition called Peyronie’s disease, in which the penis is curved

  • Heart issues like angina, narrowing of the aortic valve, irregular heartbeat or heart failure, or have previously had a stroke or heart attack

  • Blood cell-related conditions, such as multiple myeloma, sickle cell anemia or leukemia

  • High blood pressure, low blood pressure or high blood pressure in the blood vessels supplying your lungs (a condition called pulmonary arterial hypertension, or PAH)

  • An eye-related condition, such as retinitis pigmentosa (a condition in which the cells of your retina break down over time), which can increase your risk of other vision problems when using ED medications

  • Bleeding problems or stomach ulcers

  • Liver and kidney problems, including being on dialysis

When you’re dealing with ED, it can understandably trigger a range of emotions. Frustration, anxiety, depression and shame can creep into more than just your bedroom, impacting your everyday quality of life.

It may even feel tempting to throw caution to the wind and try anything just to get some sense of control back. However, it’s critical to evaluate the situation and understand your options.

Sildenafil is highly effective and safe when used as directed, but that doesn’t mean you should disregard potential risks. 

To reduce your risk of side effects and interactions, make sure you adhere to the following tips for how to take Viagra for the best results

  • Take it as prescribed. Sildenafil is most commonly prescribed to be taken one time per day in doses between 25 and 100 mg for ED. Your healthcare provider will determine the best dosage for you based on a number of factors. More does not necessarily equal better, and it’s important to stick to your prescription. 

  • Use it strategically. Sildenafil is most effective when taken at least 30 minutes to one hour before sex. For most people, it can provide ED relief for around four hours before it begins to decrease in the body. Take this into account when you’re planning to use sildenafil for sex. 

  • Know the potential side effects of sildenafil. Sildenafil isn’t for everyone. It can interact with certain medications, like nitrates, and put your health at risk. It may also lead to side effects for some men, like dizziness, prolonged erection, or visual changes. 

  • Communicate with your provider. We know how awkward it can be to talk about your sexual activity with, well, most people, but especially someone you may not see very often. It’s important to make sure your healthcare provider is aware of any preexisting health conditions you have, as well as any supplements or over-the-counter, prescription and recreational drugs you use.

If you’re unsure how to take sildenafil properly or whether it’s even right for you, consider speaking to one of our many health professionals available to provide medical advice. 

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Yes, sildenafil is usually a safe and effective drug for ED management when it’s taken as prescribed. Just keep the following things in mind if you’re considering using it to improve your sex life. 

  • Understand potential side effects. Viagra can cause side effects, interact with certain medications and be a bad option for people with certain health conditions. Speak openly with your healthcare provider about your lifestyle, existing conditions, and other drug use before starting Viagra, or any other erectile dysfunction medications.

  • Know there are other options. In some cases, another ED drug — such as Cialis or its generic tadalafil, and Stendra or its generic avanafil — may be a better fit for you. Alternatively, you might be interested in our hard mints, which are chewable ED medications that contain the same active ingredients as other ED prescriptions, but in personalized dosages.

  • Identify other factors that may be causing your ED. ED affects an estimated 30 million men in the United States alone. It’s a multifactorial condition and many men experience psychological ED. This is when things like depression, anxiety or sexual performance anxiety get in the way of your erection. And it’s more than mind over matter — these are real issues that require real attention.

ED is a cock block for everyone involved. Fortunately, living with ED doesn’t need to be a life sentence, nor is it uncommon. 

If you’re ready to pursue a solution and gain back confidence in the bedroom, check out our online therapy support or take our free quiz to help you navigate next steps.

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Editorial Standards

Hims & Hers has strict sourcing guidelines to ensure our content is accurate and current. We rely on peer-reviewed studies, academic research institutions, and medical associations. We strive to use primary sources and refrain from using tertiary references. See a mistake? Let us know at [email protected]!

This article is for informational purposes only and does not constitute medical advice. The information contained herein is not a substitute for and should never be relied upon for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment. Learn more about our editorial standards here.

Mike Bohl, MD

Dr. Mike Bohl is a licensed physician, a Medical Advisor at Hims & Hers, and the Director of Scientific & Medical Content at a stealth biotech startup, where he is involved in pharmaceutical drug development. Prior to joining Hims & Hers, Dr. Bohl spent several years working in digital health, focusing on patient education. He has also worked in medical journalism for The Dr. Oz Show (receiving recognition for contributions from the National Academy of Television Arts and Sciences when the show won Outstanding Informative Talk Show at the 2016–2017 Daytime Emmy® Awards) and at Sharecare. He is a Medical Expert Board Member at Eat This, Not That! and a Board Member at International Veterinary Outreach.

Dr. Bohl obtained his Bachelor of Arts and Doctor of Medicine from Brown University, his Master of Public Health from Columbia University, and his Master of Liberal Arts in Extension Studies—Journalism from Harvard University. He is currently pursuing a Master of Business Administration and Master of Science in Healthcare Leadership at Cornell University. Dr. Bohl trained in internal medicine with a focus on community health at NYU Langone Health.

Dr. Bohl is Certified in Public Health by the National Board of Public Health Examiners, Medical Writer Certified by the American Medical Writers Association, a certified Editor in the Life Sciences by the Board of Editors in the Life Sciences, a Certified Personal Trainer and Certified Nutrition Coach by the National Academy of Sports Medicine, and a Board Certified Medical Affairs Specialist by the Accreditation Council for Medical Affairs. He has graduate certificates in Digital Storytelling and Marketing Management & Digital Strategy from Harvard Extension School and certificates in Business Law and Corporate Governance from Cornell Law School.

In addition to his written work, Dr. Bohl has experience creating medical segments for radio and producing patient education videos. He has also spent time conducting orthopedic and biomaterial research at Case Western Reserve University and University Hospitals of Cleveland and practicing clinically as a general practitioner on international medical aid projects with Medical Ministry International.

Dr. Bohl lives in Manhattan and enjoys biking, resistance training, sailing, scuba diving, skiing, tennis, and traveling. You can find Dr. Bohl on LinkedIn for more information.


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  • Gebhart, J. J., Weinberg, D. S., Bohl, M. S., & Liu, R. W. (2016). Relationship between pelvic incidence and osteoarthritis of the hip. Bone & joint research, 5(2), 66–72. https://boneandjoint.org.uk/Article/10.1302/2046-3758.52.2000552

  • Gebhart, J. J., Bohl, M. S., Weinberg, D. S., Cooperman, D. R., & Liu, R. W. (2015). Pelvic Incidence and Acetabular Version in Slipped Capital Femoral Epiphysis. Journal of pediatric orthopedics, 35(6), 565–570. https://journals.lww.com/pedorthopaedics/abstract/2015/09000/pelvic_incidence_and_acetabular_version_in_slipped.5.aspx

  • Islam, A., Bohl, M. S., Tsai, A. G., Younesi, M., Gillespie, R., & Akkus, O. (2015). Biomechanical evaluation of a novel suturing scheme for grafting load-bearing collagen scaffolds for rotator cuff repair. Clinical biomechanics (Bristol, Avon), 30(7), 669–675. https://www.clinbiomech.com/article/S0268-0033(15)00143-6/fulltext

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