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Last updated September 24, 2020. 5 minute read

Sildenafil 20 mg: is this an effective dose for ED?

As a rule of thumb, it’s best to take the lowest effective dose of a drug. Sildenafil being available in 20 mg increments can make it easier to work with a healthcare provider to find the dose that’s best for you—one that maximizes sildenafil’s benefits while minimizing potential side effects.

Self Written by Michael Martin
Reviewed by Yael Cooperman, MD

Sildenafil is the generic form of Viagra, a.k.a. “the little blue pill,” an oral medication used to treat erectile dysfunction (ED). It’s also sold under the brand name Revatio, which is prescribed to treat pulmonary arterial hypertension (PAH), a condition in which the pressure in blood vessels that carry blood from your heart to your lungs is higher than normal (Barnett, 2006). Viagra was originally developed to treat high blood pressure until researchers discovered a not-unwelcome side effect: Patients taking it got more erections.


  • Sildenafil (brand name Viagra) is a popular medication most commonly used to treat erectile dysfunction (ED). 
  • It’s also sold under the brand name Revatio to treat pulmonary arterial hypertension (PAH).
  • Viagra is available in 25 mg, 50 mg, and 100 mg doses. Revatio is available in 20 mg, 40 mg, 60 mg, 80 mg and 100 mg doses.
  • A healthcare provider might prescribe sildenafil 20 mg (or increments thereof) to treat ED.
  • Sildenafil 20 mg can give you and a healthcare provider more options to determine the best dose of sildenafil for you.

Sildenafil is FDA-approved for ED in doses of 25 mg, 50 mg, and 100 mg (DailyMed, 2020).

So what about 20mg sildenafil? That’s the dose of Revatio that’s FDA-approved and prescribed for PAH (DailyMed, 2018). Sildenafil 20 mg is prescribed “off-label” for ED. It can be dispensed at 20 mg, 40 mg, 60 mg, 80 mg, and 100 mg. 


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Why would you want to take 20 mg sildenafil when 25 mg is available? Here are some possible scenarios:

  • Your healthcare provider may want to start you at 20 mg and see how it works.
  • 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 the slightly larger dose is better.

As a rule of thumb, it’s best to take the lowest effective dose of a drug. Sildenafil being available in 20 mg increments can make it easier to work with a healthcare provider to find the dose that’s best for you—one that maximizes sildenafil’s benefits while minimizing potential side effects. 

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How long does sildenafil 20 mg last? 

Sildenafil starts working within 30 to 60 minutes, so you should take it at least that long—and up to four hours—before sexual activity. For some men, Viagra can start working more quickly. Sildenafil reaches peak levels in the body after 30 to 120 minutes (Eardley, 2002). It can last in the body for four to five hours, but factors like your age and overall health can affect how long it will work. 

What is Viagra used for?

Viagra (sildenafil) is an oral medication for erectile dysfunction (ED). It’s part of a class of drugs called PDE5 inhibitors, which also include 

  • Vardenafil (brand name Levitra)
  • Tadalafil (brand name Cialis)
  • Avanafil (brand name Stendra)

Sildenafil and other PDE5 inhibitors 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, causing blood vessels to contract and blood to flow out of the penis. When PDE5 is inhibited, blood vessels in the penis stay dilated. This can make an erection last longer or make it easier for you to get an erection.

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It’s important to know that sildenafil doesn’t automatically give you an erection: You have to be sexually aroused for sildenafil and other PDE5 inhibitors to work.

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; both should be taken 30 minutes before sex and are effective for up to 36 hours.

What is ED?

Erectile dysfunction (ED) is when you can’t get or maintain an erection sufficient for satisfying sex. That includes erections that aren’t as firm or don’t last as long as you’d like.

ED is the most common sexual dysfunction. Experts believe more than 30 million American men have experienced ED at some point (Nunes, 2012). ED can happen at any age, but it becomes more common as men get older. 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 (Ferrini, 2017).

That said, ED is not considered a “natural” part of aging. It can be caused by a serious health condition, including:

  • Heart disease
  • Diabetes
  • High blood pressure
  • High cholesterol
  • Low testosterone
  • Nerve damage
  • Depression
  • Sleep apnea

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 before they become severe.

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Besides oral medications, treatments for ED include 

  • medications that are injected into the penis, causing an erection (alprostadil, BiMix, TriMix)
  • lifestyle changes such as improved diet, regular exercise, avoiding tobacco, drinking less alcohol, and losing weight
  • surgery to resolve issues that can cause ED, such as Peyronie’s disease
  • devices such as cock rings and penis pumps
  • counseling to resolve anxiety, depression, or other issues that might be contributing to ED

Read everything you need to know about erectile dysfunction here.

Buy Viagra

Viagra (sildenafil) is not available over the counter. You need to obtain a prescription for it. That’s because sildenafil can cause serious and even deadly side effects, and it shouldn’t be taken with certain medications.

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 have the eye condition retinitis pigmentosa, check with your healthcare provider before using those prescriptions.) Sildenafil can cause priapism, an erection lasting more than four hours. 

Sildenafil and other PDE5 inhibitors should never be taken with nitrates. These include prescription medications for heart disease and a recreational drug called poppers. Taking sildenafil with nitrates can cause a dangerous drop in blood pressure that could be fatal.

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.). 

So if you’re interested in taking sildenafil to resolve ED, do your due diligence. Check in with a healthcare provider and get a prescription from a reputable source. Your sex life, and overall health, are worth it.