Until the late 1990s, there were no truly effective oral medications for erectile dysfunction — the inability to achieve or sustain an adequate erection for sexual activity. The useful drugs that were available had to be injected into the penis or inserted into the urethra.

Treatment of erectile dysfunction is much easier now, thanks to a class of drugs called phosphodiesterase-5 (PDE-5) inhibitors. The medications — sildenafil (Viagra), vardenafil (Levitra) and tadalafil (Cialis) — all improve erectile function in the same basic way, but they differ in how quickly they take effect and how long their effects last.

How Viagra, Levitra and Cialis work

Viagra, Levitra and Cialis prevent the breakdown of nitric oxide, a chemical messenger that promotes relaxation and opening of the blood vessels that supply erectile tissue in the penis. Under the influence of nitric oxide, these vessels expand and stay dilated. Increased blood flow makes erectile tissue swell and compress the veins that carry blood out of the penis, resulting in a full erection.

PDE-5 inhibitors do not automatically trigger erections. Sexual stimulation also is needed to start the whole process. The medications enable a more complete response to sexual stimulation.

Many clinical trials have shown that PDE-5 inhibitors improve erectile function regardless of the underlying cause or causes. Viagra, Levitra and Cialis increase the number and quality of erections and sexual experiences in men with erectile dysfunction due to arteriosclerosis, diabetes, spinal cord injury, depression or the aftereffects of prostate cancer surgery.

Similarities and differences

The drugs have slightly different chemical compositions that affect how quickly they work and wear off. Other distinctions — for example, which drug may be best for men of different ages or with different medical conditions — aren’t known. No study has directly compared these three medications.

Several factors may affect your choice of medication, including how well your body responds to one drug over another and how long you want the effects of each dose to last. Talk to your doctor about your options and personal preferences to help decide which one of these medications might work for you.

Not safe for everyone

Although these medications can help many people, not all men can safely take them. PDE-5 inhibitors may worsen certain medical conditions and interact with a number of drugs. Erectile dysfunction medications are dangerous when used with nitrate medications, such as nitroglycerin (Nitro-Dur, others), often prescribed to prevent or treat acute angina (chest pain due to coronary artery disease). Both types of medication dilate blood vessels, and their combined effects can cause dizziness, low blood pressure and loss of consciousness.

In rare cases, men using PDE-5 inhibitors have suffered permanent, total vision loss due to nonarteritic anterior ischemic optic neuropathy (NAION). Because NAION and erectile dysfunction share many of the same risk factors, it’s unclear whether the drug or an underlying condition is responsible. If you’re considering an erectile dysfunction drug but have a retinal disorder, such as diabetic retinopathy, see your eye doctor first.

Realistic expectations

Occasional erectile dysfunction is a common problem, particularly as men age. Medication won’t make you feel like you’re 20 again, but it can help you achieve satisfying sexual relations more often. Be willing to work with your doctor to find which medication and dosage is best for you.

Brand name — Chemical name
Viagra — sildenafil citrate
Levitra — vardenafil
Cialis — tadalafil

Phosphodiesterase-5 inhibitors (PDE-5 inhibitors) are taken at least an hour before you plan to be sexually active. PDE-5 inhibitors will result in an erection only if you are sexually stimulated.

How It Works

During an erection, blood fills tissue in the penis, causing it to enlarge and become stiff. The PDE-5 inhibitors relax smooth muscle, allowing the penis to fill with blood.

Stimulation of the penis is necessary for Levitra and Cialis to work; Viagra can produce a spontaneous erection in those men for whom it is effective.

Why It Is Used

PDE-5 inhibitors can be used when an erection problem (erectile dysfunction) is caused by:

• Diabetes.
• High blood pressure.
• Spinal cord injury or other problems affecting nerve function.
• Prostate surgery.
• Side effects of medications.
• Emotional or psychological causes.

How Well It Works

PDE-5 inhibitors are effective treatment for erection problems. A review of trials has shown that 83% of men with erection problems who used Viagra had improved erections that allowed them to have intercourse. Initial studies show Levitra and Cialis have rates of success similar to those of Viagra. Viagra has been studied extensively and used by millions of men; its side effects and long-term effects are well recognized. Levitra and Cialis are new drugs in which long-term effects have not been studied.

PDE-5 inhibitors should be taken at least an hour before you plan to be sexually active.

One study found that Cialis, called “the weekend pill,” enhanced the ability to have erections for 24 to 36 hours after taking it. Viagra and Levitra usually enable a man to get an erection at any time for 4 or more hours after taking it.

Viagra does not work as well as it would otherwise if you eat a high-fat meal around the time you take it, so many doctors recommend you take it on an empty stomach. Levitra may be slightly less effective if you eat a high-fat meal, but a moderate-fat meal does not reduce its effectiveness; Cialis works without regard to what you eat.

Side Effects

Do not take PDE-5 inhibitors if you are taking any medication that contains nitrates, such as nitroglycerin. This combination can lower blood pressure dangerously, possibly leading to a stroke, a heart attack, or death. If you are taking a medication called an alpha-blocker—used to treat high blood pressure or an enlarged prostate—check with your health professional before you take a PDE-5 medication.

Other side effects are fairly uncommon when taking PDE-5 inhibitors. They include:

• Headache.
• Skin flushing.
• Indigestion.
• Nasal congestion.

A few men find that PDE-5 inhibitors affect their vision for a few hours after taking it, usually by causing a slight bluish tinge to their vision. A few men become more sensitive to light. These effects go away in a few hours.

What To Think About

Health experts have debated the use of PDE-5 inhibitors in men with heart disease, because deaths have been reported in men who used Viagra. The 1999 American College of Cardiology/American Heart Association (ACC/AHA) Expert Consensus Document noted that PDE-5 inhibitors may be dangerous for people who: ⁵

• Have coronary artery disease (CAD).
• Have heart failure and low blood pressure.
• Are taking many different drugs for high blood pressure.

However, several studies have reported that some men with heart problems may be able to take PDE-5 inhibitors safely.

• One study found no evidence of increased risk of heart attack or CAD in men who use Viagra.
• Another study in men with CAD reported that Viagra does not lead to heart attack and that heart attacks and other cardiovascular problems reported after taking Viagra may be related more to the physical activity of intercourse than to the medication.
• A study of 35 men found that Viagra is safe for men with moderate heart failure.

Sexual activity is exercise. If you have a heart condition and have not been sexually active for a while, talk with your health professional to make sure you can safely engage in sexual activity.

PDE-5 inhibitors are not prescribed for anyone who may take nitroglycerin in an emergency.

Tell your health professional if you are using PDE-5 inhibitors and are going to have a test for heart disease (such as a stress treadmill test, cardiac catheterization, stress thallium, or stress echocardiography). You should avoid taking PDE-5 inhibitors for 24 to 48 hours before such a test.

When considering PDE-5 inhibitors for erection problems, it is important to include your partner in your discussions.

If you take any medications, discuss PDE-5 inhibitors with your health professional before using them.

PDE-5 inhibitors can be prescribed by any doctor. You do not have to see a doctor who specializes in sexual health. PDE-5 inhibitors are also available on the Internet; if you obtain the medication through this route, you should discuss its use with your doctor.