Sildenafil 20 mg: is it an effective dosage?

Reviewed by Chimene Richa, MD, 

Written by Michael Martin 

Reviewed by Chimene Richa, MD, 

Written by Michael Martin 

last updated: Dec 20, 2022

5 min read

Sildenafil 20 mg is a dosage approved to treat a lung condition called pulmonary arterial hypertension (PAH), but it’s also prescribed to treat erectile dysfunction (ED). Sildenafil is the active ingredient in Viagra (generic Viagra is called sildenafil), a popular ED medication usually prescribed in doses of 25 mg, 50 mg, or 100 mg. Healthcare providers sometimes also prescribe sildenafil 20 mg to treat erectile dysfunction if a low dosage is the best treatment option for that person. 

Continue reading to learn if sildenafil 20 mg is the right treatment for you. 

Viagra Important Safety Information: Read more about serious warnings and safety info.

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What is sildenafil 20 mg?

Sildenafil (brand name Revatio) is typically used as a treatment for a lung condition called pulmonary arterial hypertension (PAH). The medication works by relaxing blood vessels to improve blood flow. Consequently, sildenafil can be used to treat erectile dysfunction for the same reason. Improved blood flow to the penis means harder erections for more satisfying sex.

The FDA approved sildenafil 20 mg for the treatment of PAH and the 25 mg, 50 mg, and 100 mg dosages for the treatment of erectile dysfunction. Because sildenafil is the active ingredient in Viagra, the 20 mg dose can be prescribed off-label to treat ED (DailyMed, 2020).

Why use sildenafil 20 mg?

If your healthcare provider is considering prescribing sildenafil for the treatment of ED, the traditional starting dose is 50 mg. But this dose might not be effective for everyone, and a higher dose may mean a higher risk of side effects for some people. Since generic Viagra is only available in dosages of 25 mg, 50 mg, and 100 mg, the prescribing options are limited. Clinicians can opt to prescribe 20 mg sildenafil (or multiples of 20 mg) to give their patients a more precise dose. 

Money is also a consideration when deciding between Viagra or sildenafil. Brand name Viagra is expensive and can cost upwards of $70 per dose. Opting for the generic Viagra (sildenafil) lowers the price, and opting for generic Revatio in 20 mg increments may be an even more pocket-friendly option.

So why would you want to take 20 mg sildenafil when 25 mg is available? Here are some possible reasons: 

  • Your healthcare provider may want to start you at 20 mg to see how you respond to the medication.

  • You might experience side effects on 25 mg or 50 mg, but less so (or not at all) on 20 mg or 40 mg. 

  • 50 mg might not be as effective as 60 mg for you, and you and your health provider agree a slightly higher dose is better.

As a rule of thumb, it's best to take the lowest effective dose of a drug in order to avoid potential side effects. Prescribing sildenafil in 20 mg increments can make it easier for your healthcare provider to find the dose that's best for you—one that maximizes sildenafil's benefits while minimizing potential side effects.

Sildenafil 20 mg: How long does it last? 

Sildenafil starts working within 30–60 minutes, and typically works for about four hours, so you should plan accordingly. Keep in mind that the drug doesn’t instantly give you an erection. Instead, it helps you get and maintain an erection when you get aroused (Smith, 2022). 

If you’ve taken a dose of Viagra and it doesn’t work, do not take another dose before 24 hours have passed. If you find that the dosage you’ve been prescribed isn’t effective, talk to your healthcare provider about trying a higher dose or a different medication altogether. 

Viagra alternatives

While Viagra is typically an effective treatment for erectile dysfunction, there are other similar medications that might be better treatment options for you. Other ED medications include: 

Sildenafil and other ED medications are PDE5 inhibitors, which work by suppressing an enzyme called PDE5, a.k.a. an erection's "off switch." The "on switch" is a natural chemical called cGMP, which tells blood vessels in the penis to dilate and fill with blood. PDE5 breaks down cGMP, allowing blood to flow out of the penis. When PDE5 is blocked, blood vessels in the penis stay dilated. This makes erections stay harder for longer (Dhaliwal, 2022).

Although PDE5 inhibitors all work on the same natural chemical—PDE5—that doesn't mean they're all the same. Sildenafil may take 30 minutes to work and may last for 4 to 5 hours. Avanafil can be taken 15 minutes before sexual activity, and it remains effective for about the same time as sildenafil. Tadalafil comes in two formulations: one is taken daily, the other as needed and can be effective for up to 36 hours.

Picking the right medication for you might involve a little bit of back and forth with your provider, but if a medication isn’t working as well as you would like, let them know. Getting the right medication and the right dosage can improve your sex life significantly.

Cialis Important Safety Information: Read more about serious warnings and safety info.

What causes ED?

ED—when you can't get or maintain an erection sufficient for satisfying sex—is the most common sexual dysfunction in men. Experts believe that around 30–50 million American men have experienced ED at some point. ED can happen at any age, but it becomes more common as men age. By the time a man is in his 40s, he has about a 40% chance of having experienced ED at some point. That likelihood increases by about 10% for each successive decade of life (Sooriyamoorthy, 2022; Ferrini, 2017).

That said, ED is not considered a "natural" part of aging. It’s important to know that many different underlying health conditions can contribute to it, and that addressing those conditions can often help improve symptoms of ED. These conditions include:

That's why it's important to check in with a healthcare provider at the first sign of ED. They'll evaluate your medical history, gauge your overall health, and can catch any potential health issues so you can get the treatment you need.

How to get Viagra (sildenafil)

Viagra (sildenafil) is not available over the counter, but you can consult with a healthcare provider in person or online to get a prescription. There are more options available than ever to receive Viagra discreetly in the mail, if this is a concern. 

When buying sildenafil online, beware of counterfeiters. You've seen them: At some point in your web-surfing history, you've probably wandered into an explosion of pop-up ads hawking Viagra for mere cents on the dollar. Many of the establishments behind these ads are not legit, and the product they sell can be dangerous.

In 2011, Pfizer, the manufacturer of Viagra, investigated counterfeits by buying them online and testing their contents. Some of the pills they received contained blue printer ink, amphetamines ("speed"), an antibiotic called metronidazole, too much of the active ingredient sildenafil (or not enough), and drywall (Pfizer, n.d.). 

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Side effects and warnings 

Side effects associated with sildenafil and other ED medications include headache, facial flushing, nasal congestion, stomach upset, backache, and (rarely) temporary impaired color vision. If you experience vision changes, chest pain, or priapism (an erection lasting more than four hours) seek medical attention immediately.  

Sildenafil and other PDE5 inhibitors should never be taken with nitrates or nitrites. These include prescription medications used to treat chest pain and a recreational drug called poppers. Taking sildenafil with nitrates can cause a dangerous drop in blood pressure that could be fatal.

If you're interested in taking sildenafil to treat ED, make an appointment with your healthcare provider. Your provider will help you find a medication and dosage that works for you. 


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

How we reviewed this article

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.

Current version

December 20, 2022

Written by

Michael Martin

Fact checked by

Chimene Richa, MD

About the medical reviewer

Dr. Richa is a board-certified Ophthalmologist and medical writer for Ro.

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