How Viagra Works in Your System

By Sarah Malka, MD
Medically reviewed
November 16, 2021

Viagra (sildenafil) is a brand-name prescription medication that treats erectile dysfunction (ED) by increasing blood flow to the penis.

Sometimes referred to as the “little blue pill,” Viagra is the first and oldest PDE5 inhibitor approved for patient use by the U.S. Food and Drug Administration.

When a person with ED takes Viagra, they can often achieve and sustain an erection firm enough to enjoy sexual activity.

It is one of the most popular drugs for ED on the market today.

Erectile dysfunction, or impotence, is a common sexual health condition, particularly among men as they age.

According to some estimates, 30 million men in the United States experience difficulties getting an erection or keeping one long enough to have gratifying sexual intercourse.

Roughly 70% of men will develop ED by the time they turn 70.

A healthy sex life is an integral part of your overall health and wellbeing.

Although erectile dysfunction is common, you don’t have to resign yourself to unsatisfactory sexual experiences; medical help is available.

If you are struggling to develop or maintain a firm erection, talk to a health care professional.

They will evaluate the severity of your erectile dysfunction, walk you through treatment options, and discuss whether Viagra or another ED medication might be the best way to improve your sex life.

Get erectile dysfunction prescriptions for as low as $0.90 per dose.

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What is Viagra?

Viagra (sildenafil) is a prescription drug with an active ingredient that promotes increased blood flow to the penis.

It is one of a group of medicines known as phosphodiesterase type 5 inhibitors  (PDE5 inhibitors) available to patients in the United States.

The others include Levitra (vardenafil), Cialis (tadalafil), and Stendra (avanafil).

Viagra is a brand-name medication; the drug also comes in a generic form called sildenafil.

Pfizer initially developed Viagra to help treat heart problems like angina, a type of chest pain caused by heart disease.

When they began clinical trials, however, researchers realized that the drug was positively affecting those who usually experienced challenges achieving or maintaining their erections during moments of sexual arousal.

Once it was FDA-approved in 1998, it revolutionized the way that doctors treated patients with ED.

Over the last two decades, doctors have prescribed Viagra to more than 64 million men globally.

Viagra comes in a blue pill form and can be prescribed at three different doses: 25mg, 50mg, and 100mg.

The recommended Viagra dosage is 50mg for patients up to 65 years old.

For best results, most take their tablet an hour before they plan to engage in sexual stimulation, but patients can take it up to four hours early if they want a wider window of time before they experience the desired effect. 

Never take more than your recommended dose once in a 24-hour period.

Taking more than the recommended amount of Viagra may increase the chances of developing severe side effects. 

As with any ED prescription drug, you must combine Viagra with sexual desire to achieve an erection.

The effects of Viagra can last roughly four hours, but your erection should not.

A prolonged erection can cause permanent damage to your penis.

If you have taken Viagra and have an erection for more than four hours, go to the nearest emergency room for medical help. 

What is Viagra Used For?

Viagra (sildenafil) is one of the best treatment options for those with erectile dysfunction or impotence.

Erectile dysfunction is a medical condition that describes when a person can’t achieve or sustain an erection long enough to enjoy sexual activity.

It is exceedingly common, particularly among people as they age

This medication can help a person with ED develop and maintain an erection long enough to enjoy a more satisfying sexual experience.

Like other PDE5 inhibitors, it has an active ingredient that blocks the PDE5 enzyme from breaking down a molecule that helps the blood vessels in your penis relax and widen.

As they relax, they allow more blood to flow through them into the spongy erectile tissue of your penis called the corpora cavernosa.

When the erectile tissue is full of blood, your penis expands and stiffens, leading to an erection.

Not all people with difficulties achieving an erection will benefit from ED drugs like Viagra or sildenafil.

Sexual arousal involves different parts of the body, including the brain, heart, emotions, nerves, and muscles.

There may be a number of reasons an individual person might struggle with their sexual response, including underlying causes that you should not treat with erectile dysfunction medications.

For example, some may have physical conditions like diabetes, heart conditions, Peyronie’s disease, or prostate issues that affect their ability to achieve an erection.

Others can develop erectile dysfunction because of stress, anxiety, excessive alcohol or drug use, or other mental health challenges.

How Does Viagra Work?

If you are struggling with your sexual responsiveness and believe Viagra or generic sildenafil might help you, talk to your doctor or a licensed health care provider about your condition.

They will note your medical history and prescribe a dosage that is right for you.

Always take your medication exactly as directed by your doctor or pharmacist.

Most patients take Viagra about an hour before they plan to become sexually active, but you can take it anywhere between 30 minutes and 4 hours before sex and still feel the effects.

Always swallow the pill whole; do not crush, mash, or chew it.

Altering the medication will not make it work any faster. 

For best results, take Viagra on an empty stomach.

If you must eat, avoid ingesting a large meal, a high-fat meal, or alcohol, as these can interfere with how your body processes the medication and dilute Viagra’s effectiveness.

When it Kicks in (How to Tell it’s Working)

Like others in its class of drugs, Viagra only works if you combine it with sexual arousal.

Once stimulated, you should experience a firm, sustained erection within a one-to-two-hour period after taking your pill.

Once you are erect, you are free to enjoy sex as you normally would.

How Long it Lasts

The effects of Viagra can last anywhere between 4-8 hours after you take your pill.

Some may experience a single erection during that time; others can have more than one. 

It’s important to note that although the medication is in your body for a prolonged period, your erection should never last that long.

Any erection that lasts four or more hours can cause permanent damage to the tissue in the penis.

If you have had an erection for an extended period, you may require medical care.

Call your doctor or go to the nearest emergency room for treatment. 

Side Effects

Viagra is not for everyone.

As with most medications, it has side effects and adverse drug interactions that can harm certain patients.

Most of the potential side effects of Viagra are mild, but in rare cases, they can cause more severe health problems.

The most common side effects that patients experience while taking Viagra include:

Some also report tenderness in their stomach area, bladder pain, cloudy or bloody urine, painful urination, although those side effects are less common.

If you or someone you know has taken Viagra and is experiencing any of the following symptoms, they may be having a severe reaction to the medication.

Seek medical attention by calling 9-1-1 or going to your nearest emergency room immediately. 

  • An erection that is painful or lasts longer than four hours
  • Buzzing or ringing in the ears
  • Hearing loss
  • Low blood pressure
  • Redness or swelling in the mouth
  • Severe headache
  • Sores on the tongue or lips
  • Sudden loss of vision 
  • Trembling and shaking 
  • Trouble breathing
  • Uncharacteristic Clumsiness
  • Unexpected bleeding
  • Vomiting 

Certain medications can have negative interactions with Viagra and cause health complications.

Tell your doctor about any health conditions, prescription medications, over-the-counter treatments, and other drugs you use before taking Viagra for the first time. 

Be sure to tell your doctor if you:

  • Have experienced a heart attack or stroke 
  • Have a history of heart disease, an irregular heartbeat, or chest pain
  • Have uncontrolled high blood pressure 
  • Have eye conditions like retinitis pigmentosa or non-arteritic anterior ischemic optic neuropathy (NAION)
  • Have Peyronie’s disease
  • Have sickle cell anemia
  • Have cavernosal fibrosis
  • Have liver issues
  • Have hemophilia
  • Have leukemia
  • Have been diagnosed with stomach ulcers 
  • Take nitrates 
  • Take alpha-blockers 
  • Take methamphetamines
  • Use poppers, MDMA, or cocaine recreationally

Get erectile dysfunction prescriptions for as low as $0.90 per dose.

Get Started

When to See a Doctor

Erectile dysfunction may be common, but that doesn’t mean you need to resign yourself to less than satisfying sexual encounters.

If you regularly have issues achieving or maintaining a full erection during sexual activity, call your doctor or health care provider.

They can evaluate your symptoms, discuss your medical history, and recommend a treatment plan to enhance your sexual responsiveness and improve your quality of life.

How K Health Can Help

Did you know you can get affordable primary care with the K Health app?

Download K to check your symptoms, explore conditions and treatments, and if needed, text with a doctor in minutes. K Health’s AI-powered app is HIPAA compliant and based on 20 years of clinical data.

Frequently Asked Questions

How will you know if you need Viagra?
Having challenges achieving or sustaining an erection now and again is common, but if you are experiencing an issue that is persistent or is negatively affecting your quality of life, call your doctor or a licensed provider. They can evaluate your symptoms, diagnose you with erectile dysfunction, and prescribe a medication or another treatment plan that will fit your needs.
Will Viagra make you bigger?
Viagra helps those with erectile dysfunction achieve and sustain a normal erection. It is not a penis enhancement drug.
Can Viagra be taken with other medications?
Like many medications, Viagra can interact poorly with certain drugs and cause adverse health reactions in certain patients. Give your provider your complete medical history, including any prescription medications, over-the-counter treatments, and street drugs you may use before taking Viagra.
K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Sarah Malka, MD

Dr. Sarah Malka is a board certified emergency medicine physician with K Health. She completed her residency at Harvard Medical School.