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Erectile Dysfunction: Viagra, Levitra and Cialis to Treat Erectile dysfunction

There are only three oral drugs approved by the FDA to treat erectile dysfunction: Cialis, Levitra, and Viagra. All work by increasing the flow of blood into the penis so that when a man is sexually stimulated, he can get an erection.

Mechanism of action on Viagra

The physiologic mechanism of erection of the penis involves release of nitric oxide (NO) in the corpus cavernosum during sexual stimulation. NO then activates the enzyme guanylate cyclase, which results in increased levels of cyclic guanosine monophosphate (cGMP), producing smooth muscle relaxation in the corpus cavernosum and allowing inflow of blood. Sildenafil has no direct relaxant effect on isolated human corpus cavernosum, but enhances the effect of nitric oxide (NO) by inhibiting phosphodiesterase type 5 (PDE5), which is responsible for degradation of cGMP in the corpus cavernosum. When sexual stimulation causes local release of NO, inhibition of PDE5 by sildenafil causes increased levels of cGMP in the corpus cavernosum, resulting in smooth muscle relaxation and inflow of blood to the corpus cavernosum. Sildenafil at recommended doses has no effect in the absence of sexual stimulation.

What Are the Differences Between Cialis, Levitra, and Viagra?

Cialis, Levitra, and Viagra work by a similar mechanism to cause erections. There are subtle differences in how long the drug works and how quickly it works. Levitra works a little longer than Viagra. They both take effect in about 30 minutes. With Levitra, the effects last for about 5 hours. With Viagra, the effects last approximately 4 hours.

Cialis works a bit faster (within about 15 minutes), and the effects last much longer -- up to 36 hours in some cases.
If One of These Drugs Isn't Effective for Erectile Dysfunction, Can I Try Another?

Yes, but because these drugs work the same way, it's unlikely that you'll have success with one if you've failed to achieve an adequate erection with another.
What Precautions Should I Take Before Taking One of These Drugs?

There are certain situations in which these drugs may not be safe to take. Before taking them, tell your doctor:
If you are allergic to any drugs, including Viagra or other Erectile dysfunction medications.
About any prescription or nonprescription medications you are currently taking, including herbal and dietary supplements.
If you are scheduled for surgery, including dental surgery.
If you take nitroglycerin or a long-acting nitrate to treat chest pain. The combination of Cialis, Levitra or Viagra with these drugs can cause dangerously low blood pressure.
If you take alpha-blockers for blood pressure or prostate problems. The combination of Cialis or Levitra with these drugs can also cause dangerously low blood pressure.

In addition, always follow the directions on your prescription label carefully and ask your doctor or pharmacist to explain any part that you do not understand. Take these drugs exactly as directed. Do not take more or less or take it more often than prescribed by your doctor. Do not take these drugs more than once a day.
Who Should Not Take Cialis, Levitra or Viagra?

If you have suffered from a heart attack, stroke or life threatening arrhythmia (irregular heart rate) within the last 6 months you should discuss other options with your doctor. It is also advised to avoid these drugs if you have uncontrolled high or low blood pressure or if you experience chest pain with sex.
What Are the Side Effects of Cialis, Levitra and Viagra?

Side effects are not common but they can occur. Side effects can include:
Headache
Upset stomach or ‘heartburn’
Flushing (feeling warm)
Nasal congestion
Changes in vision (color, glare)
Back pain (with Cialis)

Warning

There is a potential for cardiac risk of sexual activity in patients with preexisting cardiovascular disease. Therefore, treatments for erectile dysfunction, including VIAGRA, should not be generally used in men for whom sexual activity is inadvisable because of their underlying cardiovascular status. VIAGRA has systemic vasodilatory properties that resulted in transient decreases in supine blood
pressure in healthy volunteers (mean maximum decrease of 8.4/5.5 mmHg), (see CLINICAL PHARMACOLOGY: Pharmacodynamics). While this normally would be expected to be of little consequence in most patients, prior to prescribing VIAGRA, physicians should carefully consider
whether their patients with underlying cardiovascular disease could be affected adversely by such vasodilatory effects, especially in combination with sexual activity. Patients with the following underlying conditions can be particularly sensitive to the actions of vasodilators including Viagra – those with left ventricular outflow obstruction (e.g. aortic stenosis, idiopathic hypertrophic subaortic stenosis) and those with severely impaired autonomic control of blood pressure.