The common PDE5 drugs approved in the United States are sildenafil (Viagra), vardenafil (Levitra), or tadalafil (Cialis). Actual head-to-head trials between these drugs have not been done to date to see which is the superior drug. Details on each of these medications for erectile dysfunction are outlined below.
Sildenafil (Viagra) was the first oral phosphodiesterase type 5 (PDE5) inhibitor approved by the FDA in the United States for the treatment of erectile dysfunction (it is not approved for women). Sildenafil inhibits PDE5, which is an enzyme that destroys cGMP. By inhibiting the destruction of cGMP by PDE5, sildenafil allows cGMP to accumulate. The cGMP in turn prolongs relaxation of the smooth muscle of the corpora cavernosa. Relaxation of the corpora cavernosa smooth muscle allows blood to flow into the penis resulting in increased engorgement of the penis. In short, sildenafil increases blood flow into the penis and decreases blood flow out of the penis.
How effective is sildenafil (Viagra)?
Sildenafil is used for the treatment of erectile dysfunction of either physical or psychological cause. It has been found to be effective in treating erectile dysfunction in men with coronary artery disease, diabetes mellitus, hypertension, depression, coronary artery bypass surgery, and men who are taking antidepressants and several classes of anti-hypertensives.
In randomized controlled trials, an estimated 60% of men with diabetes, and 80% of men without diabetes experienced improved erections with sildenafil.
How should sildenafil (Viagra) be administered?
Sildenafil is available as oral tablets at doses of 25, 50, and 100 mg. It should be taken approximately one hour before sexual activity. In some men, the onset of action of the drug may be as early as 11-20 minutes. Sildenafil should be taken on an empty stomach for best results since absorption and effectiveness of sildenafil can be diminished if it is taken shortly after a meal, particularly a meal that is high in fat.
What is the dose of sildenafil (Viagra)?
In prescribing sildenafil, a doctor considers the age, general health status, and other medication(s) the patient is taking. The usual starting dose for most men is 50 mg, however, the doctor may increase or decrease the dose depending on side effects and effectiveness. The maximum recommended dose is 100 mg every 24 hours, however, many men will need 100 mg of sildenafil for optimal effectiveness, and some doctors are recommending 100 mg as the starting dose.
Metabolism (breakdown) of sildenafil is slowed by aging, liver and kidney dysfunction, and concurrent use of certain medications (such as erythromycin -- an antibiotic, and protease inhibitors, for HIV). Slowed breakdown allows sildenafil to accumulate in the body and potentially may increase the risk of side effects. Therefore in men over 65, in men with substantial kidney and liver disease, and in men who also are taking protease inhibitors, the doctor will initiate sildenafil at a lower dose (25 mg) to avoid accumulation of sildenafil in the body. A protease inhibitor ritonavir (Norvir) is especially potent in increasing the accumulation of sildenafil, thus men who are taking Norvir should not take sildenafil doses higher than 25 mg and at a frequency of no greater than once in 48 hours.
What are the side effects of sildenafil (Viagra)?
Sildenafil has been found to be well tolerated without important side effects. The reported side effects are usually mild and include headache, flushing, nasal congestion, nausea, dyspepsia, (stomach discomfort), diarrhea, and abnormal vision (seeing a bluish hue or brightness).
Sildenafil can cause hypotension (abnormally low blood pressure that can lead to fainting and even shock) when given to patients who are taking nitrates (for heart disease). Therefore, patients taking nitrates daily should not take sildenafil. Nitrates are used most commonly to relieve angina (chest pain due to insufficient blood supply to the heart muscle because of narrowing of the coronary arteries); these include nitroglycerine tablets, patches, ointments, sprays, and pastes, as well as isosorbide dinitrate, and isosorbide mononitrate. Other nitrates such as amyl nitrate and butyl nitrate also are found in some recreational drugs called "poppers."
Sildenafil should be used cautiously in men on alpha blockers such as doxazosin (Cardura), terazosin (Hytrin), and tamsulosin (Flomax). There have been occasional reports of low blood pressure in men who have taken the two classes of drugs simultaneously and therefore it is recommended that there be at least a span of four to six hours between the ingestion of sildenafil and alpha blockers.
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