Viagra, Levitra, and Cialis all belong to a class of drugs called phosphodiesterase (PDE) inhibitors. Taken an hour before sexual activity, these drugs work by enhancing the effects of nitric oxide, a chemical that relaxes smooth muscles in the penis during sexual stimulation and allows increased blood flow.

While oral medicines improve the response to sexual stimulation, they do not trigger an automatic erection as injections do. The recommended dose for Viagra is 50 mg, and the physician may adjust this dose to 100 mg or 25 mg, depending on the patient. The recommended dose for either Levitra or Cialis is 10 mg, and the physician may adjust this dose to 20 mg if 10 mg is insufficient. A lower dose of 5 mg is available for patients who take other medicines or have conditions that may decrease the body's ability to use the drug. Levitra is also available in a 2.5 mg dose.

None of these PDE inhibitors should be used more than once a day. Men who take nitrate-based drugs such as nitroglycerin for heart problems should not use either drug because the combination can cause a sudden drop in blood pressure. Also, tell your doctor if you take any drugs called alpha-blockers, which are used to treat prostate enlargement or high blood pressure. Your doctor may need to adjust your ED prescription. Taking a PDE inhibitor and an alpha-blocker at the same time (within 4 hours) can cause a sudden drop in blood pressure.

Oral testosterone can reduce ED in some men with low levels of natural testosterone, but it is often ineffective and may cause liver damage. Patients also have claimed that other oral drugs—including yohimbine hydrochloride, dopamine and serotonin agonists, and trazodone—are effective, but the results of scientific studies to substantiate these claims have been inconsistent. Improvements observed following use of these drugs may be examples of the placebo effect, that is, a change that results simply from the patient's believing that an improvement will occur.

Many men achieve stronger erections by injecting drugs into the penis, causing it to become engorged with blood. Drugs such as papaverine hydrochloride, phentolamine, and alprostadil (marketed as Caverject) widen blood vessels. These drugs may create unwanted side effects, however, including persistent erection (known as priapism) and scarring. Nitroglycerin, a muscle relaxant, can sometimes enhance erection when rubbed on the penis.

A system for inserting a pellet of alprostadil into the urethra is marketed as Muse. The system uses a prefilled applicator to deliver the pellet about an inch deep into the urethra. An erection will begin within 8 to 10 minutes and may last 30 to 60 minutes. The most common side effects are aching in the penis, testicles, and area between the penis and rectum; warmth or burning sensation in the urethra; redness from increased blood flow to the penis; and minor urethral bleeding or spotting.

Research on drugs for treating ED is expanding rapidly. Patients should ask their doctor about the latest advances.

TadalafilSide effects of the big three pharmaceutical ED prescriptions;

Viagra, Cialis, Levitra

Viagra (sildenafil)

This pharmaceutical, like the other two, works by inhibiting the enzymes that constrict the circulatory system. In most men Viagra works within an hour, lasting for about four hours. On of the problems with Viagra is that you can't take it after eating dinner as it is easily blocked by food in the stomach. And, of course, if on a date and you take it before dinner you will be sitting there with a flushed face. Other common side-effects include headache, visual disturbances, blocked nose, (making sleep after Viagra sex difficult) flushed face, indigestion, palpitations - and dizziness. So don't get out of bed too quickly. And, don't take too much as blue vision occurs at higher doses. Viagra is quite dangerous when taken with certain heart drugs. It also interacts with several medications. Do not drink grapefruit juice on day of use.

At the beginning of April 2005, a report from the University of Minnesota suggested that a small group of men have suffered blindness as a result of Viagra use. It was claimed that the drug may rarely cause a serious eye condition called ‘non-arteritic ischaemic optic atrophy.’ In 2007, it became apparent that about half a dozen British men have suffered similar eye problems. If you are taking Viagra, you should ask your doctor to keep you informed of any further research on this development.

Cialis (tadalafil)

Works in the same way as Viagra. Side-effects similar to those of Viagra, but because it is a variation on phosphodiesterase inhibitors that is held by the body in skeletal muscles it commonly causes back and general muscle pain. Interactions with other drugs and with grapefruit juice are similar to those of Viagra, but also clashes with the antibiotic clarithromycin and the sedative phenobarbitone. Cialis is very similar in structure to Viagra, but so far (2007) there have been no reports of blindness on Cialis.

Levitra (vardenafil)

Works in same way. Side-effects and interactions are similar to those of Viagra. Not as long-lasting as Cialis. So far, there have been no reports of blindness.

Drug interactions with phosphodiesterase inhibitors

Many medicines and/or medical conditions interact with phosphdiesterase inhibitors (Viagra, Cialis, Levitra). It is always prudent to talk to your doctor or pharmacist about all of the prescriptions as well as "natural" protocols that you are taking. It is a well established fact that one should not take any form of nitroglycerin, other nitrates such as isosorbide, nitroprusside or any "nitric oxide donor" medicine, or recreational drugs called "poppers" containing amyl or butyl nitrate because very serious interactions may occur. If you are not sure whether a certain medicine is a nitrate, contact your doctor or pharmacist before taking this medicine. Additional monitoring of your dose or condition may be needed if you are taking any other medicine for erectile dysfunction; certain antifungal medicines such as itraconazole or ketoconazole; macrolide antibiotics such as clarithromycin or erythromycin; medicine for high blood pressure; "water pills" such as bendrofluazide or hydrochlorothiazide; certain alpha-blocker medicines such as alfuzosin, doxazosin, prazosin, or terazosin; or HIV protease inhibitors such as ritonavir or indinavir.

Inform your doctor of any other medical conditions including penis problems; history of painful or prolonged erection; any heart problems; kidney problems; stomach ulcers; bleeding problems; blood cell problems such as sickle cell anemia, leukemia, or myeloma; eye diseases, especially retina diseases such as retinitis pigmentosa; uncontrolled high blood pressure; or allergies.

If you have a history of angina or chest pain, heart attack, stroke, low blood pressure, severe liver problems, or if you have been told by your healthcare provider to not have sexual activity because of certain health problems, using phosphodiesterase inhibitors is not recommended!

Side effects

The side effects of taking phosphodiesterase inhibitors include headache, flushing, stomach upset or nausea, heartburn, stuffy or runny nose, back pain, muscle pain, or vision changes such as trouble telling the difference between blue and green or seeing a blue tinge to objects. If these continue or are bothersome, check with your doctor.

Phosphdiasterase overdose

Contact your local poison control center or emergency room immediately. Symptoms of overdose may include back pain, muscle pain, and abnormal vision.