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Oral Medications for Erectile Dysfunction: Viagra, Cialis, and Levitra Compared


Oral medications – specifically PDE5 inhibitors – are the most common treatment for erectile dysfunction. These drugs include Viagra, Cialis, Levitra, and Stendra, and their generic equivalents (sildenafil, tadalafil, vardenafil, and avanafil, respectively).

The American Urological Association, the Canadian Urological Association, and the British Association of Urological Surgeons all recommend PDE5 inhibitors as a first-line treatment for erectile dysfunction, once reversible causes of ED are ruled out.

They are sometimes used in combination with other treatments.  Oral medications alone are effective for 65-70% of men suffering from ED.

It’s important to understand that these medications do not cure erectile dysfunction.  However, they allow many men with ED to achieve and maintain an erection.

PDE5 Inhibitors

The major oral medications for ED all operate on the same principle.  They inhibit the action of type 5 phosphodiesterase (PDE5), thereby increasing the blood flow and helping to trap the blood in the penis.  (See our article “How Do Erections Work“).

A PDE5 inhibitor prevents the phosphodiesterase type 5 enzyme from breaking down cyclic guanosine monophosphate (cGMP). This inhibition results in an increase in the concentration of penile cavernosal cGMP, causing smooth muscle relaxation in the corpus cavernosum vasculature, leading to increased erection hardness and duration in men with erectile dysfunction who have sufficient intact vasculature. 

There are several PDE-5 inhibitors available: sildenafil (Viagra), tadalafil (Cialis), avanafil (Stendra), and vardenafil (Levitra).

These drugs have been subjected to extensive clinical studies, and have been used for more than 20 years. All drugs have side-effects (see below) and precautions, but when taken as directed, PDE5 inhibitors are a safe and effective treatment option.

There is no “best” pill for erectile dysfunction. The drugs in this class are all similar, but men respond differently to each. Most doctors will start a patient with sildenafil, mostly because it’s been around the longest and they are comfortable with it.

PDE-5 inhibitors work well for about 65% of the men who take them. Men often respond better to one than to another, so it you are not getting good results, ask your doctor to switch your prescription.

Side Affects

Common side effects of these drugs may include headaches.  You can use over-the-counter pain medications to deal with this.

Less common side effects include back pain, nausea, indigestion, nasal congestion, dizziness, and blurred vision.  Some side effects can be serious! If you experience side effects, you should talk to your doctor.

There are some variations in how each medication works, so if one doesn’t work, it’s worth trying others.


PDE5 inhibitors can cause serious problems for men with certain medical conditions, including heart disease, liver disease, kidney disease, low blood pressure, or certain retinal disorders.

PDE5 inhibitors may also have dangerous interactions with other medications, including alpha blockers and nitrate medications.

It is very important to discuss your medical history and medications you are taking with your doctor before using a PDE5 inhibitor.

Danger of Overdose

Patients should be careful not to exceed the maximum recommended dosage of any PDE5 inhibitor. Overdoses can cause priapism, a painful and potentially dangerous condition in which an erection persists more than 4 hours.

Overdoses can also cause permanent retinal damage (see our news stories, “Patient Experiences Red Vision After Sildenafil Citrate Overdose” and “Retinal Damage After Sildenafil Citrate Overdose“).

The Major PDE5 Inhibitors Compared

Levitra (vardenafil)

Levitra is very similar in its effects to Viagra.  It is less affected by alcohol and food consumption.  There is some evidence that Levitra is more effective for men with diabetes.

Generics.   The patent on Levitra has expired, so it is available in the US and Canada as a generic (vardenafil), which is considerably less expensive.  The generic form has the same chemical properties as the brand name drug.

Stendra / Spedra (avanafil)

Similar to other pde5 inhibitors; may be taken with food,

Generics.   Stendra is not currently available as a generic.

Viagra (sildenafil)

Viagra is the oldest pde5 inhibitor on the market, so it is very well understood by doctors.

It takes effect in about 30 minutes; for most men the effects last 2-3 hours.  Viagra should not be taken with alcohol, or after eating (especially fatty foods), which can interfere with its effects.

Generics.  The patent on Viaga has expired, so it is available in the US and Canada as a generic (sildenafil citrate), which is considerably less expensive.  The generic form has the same chemical properties as the brand name drug.

Cialis (tadalafil)

Cialis can last up to 36 hours, so many men prefer it over Viagra or Levitra.  However, side effects (if any) will also last longer.  Because tadalafil remains in the bloodstream for a relatively long time, men may take a small dose every day. The drug gradually builds up in the body, allowing many men to achieve and erection at any time (see “Cialis for Daily Use” below).

Cialis is affected by alcohol, but it is less susceptible to food consumption than Viagra.

Generics.  The patent on Cialis has expired, so it is available in the US and Canada as a generic (tadalafil), which is considerably less expensive.  The generic form has the same chemical properties as the brand name drug.

Comparison – Cialis / Levitra / Stendra / Viagra

Sildenafil (Viagra)Tadalafil (Cialis)Vardenafil (Levitra)Avanafil (Stendra)
Dosage20-100 mg5-20 mg; 2.5-10 mg for daily use5-20 mg50-200 mg
Time to Take Effect30-60 minutes2-3 hours30-60 minutes15-30 minutes
Effects LastUp to 4 hoursUp to 36 hoursUp to 4 hoursUp to 4 hours
Maximum FrequencyOnce per dayNo more than 20 mg per 48 hoursNo more than 20 mg per dayOnce per day
Before TakingAvoid fatty foods or large meals. Avoid alcohol. Avoid grapefruit or grapefruit juice.Avoid grapefruit or grapefruit juice.Avoid fatty foods or heavy meals. Avoid grapefruit or grapefruit juice.No restrictions.

Cialis for Daily Use

Unlike other PDE5 inhibitors, Cialis (tadalafil) remains in the bloodstream for a long time. It has a “half-life” about about 17.5 hours, which means that half the original dose still remains. If you continue taking daily doses, tadalafil builds up in your body.

After 24 hours, 38% of the original dose is still in your body. If you take another pill, you’re at 138%.

After another twenty-four hours there is 52% in your body, so taking another pill will boost you to 152%.

You’ll eventually reach a steady-state where the dose that is in your bloodstream never falls below 60%.

In other words, depending on the dose that your doctor prescribes, you should be able to achieve an erection at any time. (Though obviously the results will be best shortly after you take the daily pill, when the amount in your bloodstream is at its highest.)

Getting a Prescription

PDE5 inhibitors must be prescribed by a doctor.  The doctor will:

  • determine the likely cause of your erectile dysfunction
  • make sure that the medication will not impact pre-existing medical conditions (if any)
  • make sure that the medication will not have negative interactions with other drugs you may be taking
  • recommend the best medication or your needs

Once your doctor has written a prescription, it can be filled at a local pharmacy. Be sure to compare prices! The cost of the same prescription drug can vary by hundreds of dollars at different pharmacies, even within the same town

Where to Buy ED Medications

Herbal Supplements

Herbal supplements are generally not very effective.  There may be serious risks associated with using herbal supplements.  See our article “Herbal Medications for Erectile Dysfunction.”

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