Similarities and Differences Between Cialis and Viagra

By Terez Malka, MD
Medically reviewed checkmarkMedically reviewed
November 29, 2021

Erectile dysfunction (ED), or impotence, is the inability to achieve or sustain a firm erection long enough to enjoy sexual activity.

The condition becomes more common with older age, but young men can also experience ED: One study from 2013 found that around 25 percent of erectile dysfunction patients were 40 or younger.

It’s estimated that 30 million American men suffer from ED, and previous surveys found that 70 percent of men over age 70 suffered from the condition.

ED doesn’t have to be a “normal” part of aging.

Leaving the condition untreated can lead to more than just sexual frustration.

It can also negatively impact your relationships, mental health, and overall quality of life.

Fortunately, several prescription drugs can address your symptoms and improve your erections.

Some underlying medical conditions like diabetes and heart disease can cause ED, so it is important to see your primary care provider for an evaluation before starting any ED treatment.

Two brand-name medications, Cialis (tadalafil) and Viagra (sildenafil), have been successfully used for decades to treat patients who have been diagnosed with ED.

They are both a type of medication called phosphodiesterase type 5 inhibitors, or PDE5 inhibitors.

These relax the smooth muscle in the penis and dilate the blood vessels to improve blood flow.

When combined with sexual stimulation, these ED treatments help patients achieve and maintain an erection.

Although Cialis and Viagra are in the same class of medications and work in similar ways, there are key differences between them.

They each have unique benefits, active ingredients, side effects, and drug interactions.

For that reason, patients may find that, when it comes to improving their personal experience, one medication is a better fit than the other.

In this article, I’ll explain the differences between Cialis and Viagra, including how they work, the dosage of each drug, their effectiveness, costs, side effects, and interactions with other medications.

I’ll also outline how you can get a prescription for Cialis or Viagra, and when to talk to your doctor or other healthcare professional.

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What are Cialis and Viagra?

Cialis and Viagra are two brand-name prescription medications that treat erectile dysfunction (impotence; ED), a medical condition that inhibits individuals from achieving or maintaining an erection long enough or firm enough for enjoyable intercourse.

ED is one of the most common sexual dysfunctions, particularly among patients over age 40

Both Cialis and Viagra are PDE-5 inhibitors, a type of medication that helps increase blood flow to the penis.

Both Cialis and Viagra are also available as generic medications.

These generic versions contain the same active ingredient as the brand-name version, are equally effective, and are usually sold at a lower cost.

The generic version of Cialis is called tadalafil, and generic Viagra is called sildenafil.

Cialis and Viagra are both oral medications that are considered first-line therapies in the treatment of ED.

They are both effective in helping patients achieve sexual satisfaction, and usually have fewer side effects than older, more invasive treatment options.

The active ingredients in Viagra and Cialis are also used to treat other health conditions.

Sildenafil citrate, the active ingredient in Viagra, is also marketed as Revatio when used to treat pulmonary hypertension.

Cialis is FDA-approved to treat benign prostatic hyperplasia, a condition that leads to enlarged prostate glands.

How Each Works

Both Cialis and Viagra are medications called phosphodiesterase type 5 inhibitors (PDE5 inhibitors).

They work by widening the blood vessels in the penis and relaxing penile muscles to allow enough blood flow to achieve an erection.  

While the basic mechanisms are the same, there are some important differences between the two medications.

Cialis

Cialis lasts much longer in the body than Viagra.

Cialis begins to work roughly 30 minutes after people take the medication and can last up to 36 hours in your system.

That means you can take your dose once per day and achieve an erection anytime you’d like, day or night.

Viagra

Viagra stays active in your system for about 4 hours after taking it.

It is typically taken 30-60 minutes before a sexual encounter.

It also has slightly different chances for side effects than Cialis. 

Dosage

Cialis and Viagra are oral medications that doctors or licensed healthcare providers can prescribe at various dosages depending on patient needs.

Cialis comes in a yellow, oval tablet. Viagra is a small, blue, diamond-shaped tablet.

The generic forms of these medications look different, but are usually equally effective.

Cialis

Cialis is available as a pill in four different dosage strengths: 2.5mg, 5mg, 10mg, and 20mg.

You can take Cialis once per day, or on an as-needed basis 30-60 minutes before sexual activity.

For those taking one one daily pill, providers will usually prescribe 2.5 or 5 mg per day.

For patients who are prescribed Cialis on an as-needed basis, 10 mg is the usual starting dose.

If you have taken Cialis and do not believe your dose is enough for you, talk to your prescriber before increasing your dose.

Taking more than the recommended dose can cause serious side effects. 

Viagra

Viagra is available as a pill in 25 mg, 50 mg, and 100 mg doses.

Your dosage will depend on your age and overall health, the severity of your condition, how well you respond to the drug, as well as any other medications you take.

Patients can take Viagra on an as-needed basis, but should never take more than one pill in 24 hours. 

A “normal” Viagra dose is 50 mg, which resulted in improved erections for 74 percent of men in clinical trials.

A 25 mg pill is an adequate dose for many patients, though.

On average, Viagra stays in your system for 4-8 hours, and it is most effective within the first 1-4 hours after taking it.

Depending on the patient’s experience, the dose may be adjusted higher or lower.

Effectiveness

Both Viagra and Cialis are considered first-line therapies for patients with erectile dysfunction because of their high success rate. 

Cialis

Several clinical trials suggest that Cialis is highly effective at enhancing erections and improving patient sexual experience.

In one study of 348 patients, nearly 60% of participants who took tadalafil, the active ingredient in Cialis, completed sexual intercourse.

Only 28.3% of the control group completed intercourse in the study.

Viagra

Viagra’s effectiveness has been well-documented for more than two decades.

In one study, researchers asked participants who had taken sildenafil (Viagra’s active ingredient) for 1-3 years if they were satisfied with the effect of the medication on their erections.

More than 95% answered yes. 

Cost Comparison

ED medication can be expensive. If you have been diagnosed with ED and have a prescription for either Viagra or Cialis, you can purchase your medication through a reputable online pharmacy or your local pharmacy.

Costs will vary depending on your dosage, insurance coverage, type of pharmacy, and your location.

Cialis

Depending on the strength you have been prescribed, Cialis can cost as little as $29, or more than $100 per tablet.

Many patients rely on insurance coverage or pharmacy coupons to reduce the price of their prescriptions. If you are concerned about price, the average retail cost of generic tadalafil is much lower.

It provides the same benefits as Cialis.

Viagra

Viagra can cost between $37 and $100 per pill or more, depending on the dose.

Some patients rely on insurance coverage or drug coupons to lower the price, while others seek out sildenafil, the generic form of Viagra.

The average cost of sildenafil is around $20 for 10 tablets, depending on your pharmacy.

Side Effects

Cialis and Viagra are not for everyone.

In certain patients, they can cause side effects that range from mild to severe.

If you are taking Cialis or Viagra and begin to experience any unwanted symptoms that you believe might be related to the treatment, stop taking the medication and speak with your prescriber so they can adjust your dosage or medication if necessary.

Cialis

Most patients who take Cialis experience only mild side effects, if any.

The most common mild side effects include: 

Some patients develop more serious, or even life-threatening side effects of Cialis.

If you are experiencing any of the following symptoms, stop taking Cialis and seek medical help immediately:

  • An erection that won’t go away or lasts more than four hours—this can cause permanent penis damage
  • Changes to your color vision
  • Dizziness or lightheadedness
  • Sudden vision loss in one or both eyes
  • Ringing in the ears 
  • Sudden hearing loss
  • Chest pain
  • Shortness of breath
  • Weakness or numbness

Viagra

Most people experience only mild side effects with Viagra.

The most common include: 

  • Headache
  • Red, flushed, or warm skin
  • Stomach ache or indigestion
  • Stuffy nose
  • Runny nose
  • Back pain
  • Muscle ache
  • Nausea
  • Dizziness
  • Rash

Sildenafil can also cause problems—even permanent vision loss or impairment—in patients with certain pre-existing eye conditions such as retinitis pigmentosa and non-arteritic anterior ischemic optic neuropathy (NAION).

If you are at risk for either of these conditions, talk to your ophthalmologist before beginning any ED medication. 

Patients with heart problems, including those who have experienced a heart attack, stroke, irregular heartbeat, or low blood pressure, should be very cautious before taking sildenafil or any other erectile dysfunction medication.

In rare cases, patients can develop other serious side effects while taking Viagra.

If any of these occur, discontinue your medication and seek emergency medical care:

  • An erection that won’t go away or lasts more than four hours 
  • Sudden vision loss in one or both eyes
  • Changes in color vision 
  • Blurred vision
  • Sudden hearing loss 
  • Ringing in the ears 
  • Dizziness or lightheadedness
  • Chest pain
  • Shortness of breath
  • Weakness or numbness

Drug Interactions

Cialis and Viagra can be dangerous when taken with certain other medications.

Be sure to share your complete medical history with your doctor or healthcare provider before taking an ED medication.

Tell them about any other prescription drugs, over-the-counter treatments, homeopathic remedies, or illicit drugs you regularly take before taking your first dose of Viagra or Cialis.

Cialis

The following medications and supplements may cause dangerous interactions when taken with Cialis.

  • Nitrates
  • Alpha blockers 
  • Antihypertensives
  • Certain antibiotics, including clarithromycin, erythromycin and telithromycin
  • Diltiazem
  • Itraconazole
  • Ketoconazole
  • Ritonavir
  • Verapamil
  • Goldenseal 

Never take Cialis with other PDE-5 inhibitors.

Patients can take Cialis with or without food, but alcohol should be limited.

Avoid grapefruit when taking Cialis. 

If you have any of the following health conditions in your medical history, speak with your healthcare provider before taking Cialis: 

  • Myocardial infarction 
  • Unstable angina 
  • Heart failure
  • Uncontrolled arrhythmia 
  • Hypotension
  • Uncontrolled hypertension
  • Stroke 
  • Priapism
  • Non-arteritic anterior ischemic optic neuropathy (NAION)
  • Hereditary degenerative retinal disorders (retinitis pigmentosa)
  • Liver impairment
  • Kidney impairment

Patients who have a known sensitivity to tadalafil should not take Cialis.

If you take Cialis and begin to cough or wheeze, experience swelling in your lips, face, or tongue, or begin to have trouble speaking or breathing, call 9-1-1 or go to the nearest emergency room.

You may be experiencing an allergic reaction that can be life-threatening if left untreated.

Viagra

If you are taking any of the following medications, talk to your doctor or healthcare provider before you begin using Viagra.

  • Alpha blockers
  • Antihypertensives
  • Clarithromycin
  • Diltiazem 
  • Erythromycin
  • Goldenseal
  • Itraconazole 
  • Ketoconazole
  • Nitrates
  • Ritonavir
  • Saquinavir
  • Verapamil

Never take Viagra with other PDE-5 inhibitors or EDn-related treatments.

Do not drink excessive amounts of alcohol while taking Viagra or eat heavy, rich, or high-fat meals, as they can cause the medication not to work as well.

Eating grapefruit while taking Viagra may interfere poorly with the drug.

If you have a medical history of any of the following conditions, you should take extra precautions before taking Viagra:

  • Liver impairment
  • Kidney impairment
  • Cardiac failure
  • Coronary artery disease
  • Angina
  • Hypotension
  • Hypertension
  • Stroke
  • Myocardial infarction
  • Non-arteritic anterior ischemic optic neuropathy (NAION)
  • Life-threatening Arrhythmia 
  • Priapism

Do not take Viagra if you have a sensitivity to Viagra.

If, after taking the medication, you begin to experience swelling in your tongue, mouth, face, or throat, have trouble breathing or speaking, or begin to cough or wheeze, call 9-1-1 or go to the nearest emergency room.

You might be experiencing a life-threatening allergic reaction that requires immediate medical attention.

How To Get Cialis or Viagra

Viagra and Cialis are only available by prescription.

If you would like to purchase either medication, you must first make an appointment with a doctor or licensed healthcare provider to discuss your condition and any symptoms you are experiencing.

Once your provider diagnoses you with erectile dysfunction and determines that medication will be a safe option for you, they will write you a prescription for the ED medication that you need.

If you have been diagnosed with ED and have a prescription, you will be able to purchase your medication from any reputable online pharmacy or local drug store.

Never buy your pills from an unlicensed source—counterfeit medications are not regulated and can be dangerous or fatal.

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When to See a Doctor or Healthcare Professional

Most people experience difficulties with getting erect now and again.

If you experience chronic challenges achieving or maintaining an erection that are negatively affecting your sexual or mental health, your relationship, or your quality of life, talk to your primary care provider about your symptoms.

They can evaluate your symptoms, assess for any underlying condition, and recommend the best treatment to address your concerns and improve your sexual health.

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

Can Cialis and Viagra be taken at the same time?
No. You should never take Cialis and Viagra together or combine them with any other ED medications. If you take more than one erectile dysfunction medication at a time, you won’t experience increased benefits. Instead, you are putting yourself at risk for severe, even life-threatening side effects, as well as permanent damage to your penis.
Does Viagra or Cialis work faster?
Viagra and Cialis begin to work in about the same amount of time. It takes around 30 minutes for both drugs to start to work. Viagra’s effects last about four hours, while Cialis stays in the body for up to 36 hours after a single dose.
Which is healthier for your body, Viagra or Cialis?
Viagra and Cialis are both FDA-approved prescription drugs. They are safe, proven, and effective medications when used appropriately to treat patients with erectile dysfunction.
Can you take Viagra or Cialis while pregnant?
The Food and Drug Administration has not approved Viagra and Cialis for patients who do not have a penis. Therefore, if you are pregnant, do not take Viagra or Cialis.

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.

Terez Malka, MD

Dr. Terez Malka is a board-certified pediatrician and emergency medicine physician.