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Generic Viagra

Let's take a look what Wikipedia says about Generic Viagra!

Sildenafil citrate, sold as Viagra, Revatio and under various other trade names, is a drug used to treat erectile dysfunction and pulmonary arterial hypertension (PAH). It was originally developed by British scientists and then brought to market by the US-based pharmaceutical company Pfizer.[1] It acts by inhibiting cGMP-specific phosphodiesterase type 5, an enzyme that promotes degradation of cGMP, which regulates blood flow in the penis. Since becoming available in 1998, sildenafil has been the prime treatment for erectile dysfunction; its primary competitors on the market are tadalafil (Cialis) and vardenafil (Levitra).

Medical uses:

Sexual dysfunction

The primary indication of sildenafil is treatment of erectile dysfunction (inability to sustain a satisfactory erection to complete intercourse). Its use is now standard treatment for erectile dysfunction in all settings, including diabetes.[2]

People on antidepressants may experience sexual dysfunction, either as a result of their illness or as a result of their treatment. A 2003 study showed that sildenafil improved sexual function in men in this situation.[3] Following up reports from 1999,[4] the same researchers found that sildenafil improved sexual function in female patients on antidepressants as well.[5]

Source: http://en.wikipedia.org/wiki/Viagra

Similiar info can be found on drugs.com:

What is Viagra?

Viagra (sildenafil) relaxes muscles and increases blood flow to particular areas of the body.

Viagra is used to treat erectile dysfunction (impotence) in men. Another brand of sildenafil is Revatio, which is used to treat pulmonary arterial hypertension and improve exercise capacity in men and women. Do not take Viagra while also taking Revatio, unless your doctor tells you to.

Viagra may also be used for other purposes not listed in this medication guide.

Important information about Viagra

Do not take Viagra if you are also using a nitrate drug for chest pain or heart problems. This includes nitroglycerin (Nitrostat, Nitrolingual, Nitro-Dur, Nitro-Bid, and others), isosorbide dinitrate (Dilatrate-SR, Isordil, Sorbitrate), and isosorbide mononitrate (Imdur, ISMO, Monoket). Nitrates are also found in some recreational drugs such as amyl nitrate or nitrite ("poppers"). Taking Viagra with a nitrate medicine can cause a sudden and serious decrease in blood pressure. During sexual activity, if you become dizzy or nauseated, or have pain, numbness, or tingling in your chest, arms, neck, or jaw, stop and call your doctor right away. You could be having a serious side effect of Viagra.

Source: http://www.drugs.com/viagra.html

Here's some more info on other ED tablets from Wikipedia, the free encyclopedia:

Tadalafil is a PDE5 inhibitor marketed in pill form for treating erectile dysfunction (ED) under the name Cialis, and under the name Adcirca for the treatment of pulmonary arterial hypertension. In October 2011 the U.S. Food and Drug Administration (FDA) approved Cialis for treatment the signs and symptoms of benign prostatic hyperplasia (BPH) as well as a combination of BPH and erectile dysfunction (ED) when the conditions coincide. It initially was developed by the biotechnology company ICOS, and then again developed and marketed world-wide by Lilly ICOS, LLC, the joint venture of ICOS Corporation and Eli Lilly and Company. Cialis tablets, in 5 mg, 10 mg, and 20 mg doses, are yellow, film-coated, and almond-shaped. The approved dose for pulmonary arterial hypertension is 40 mg (two 20-mg tablets) once daily.

Tadalafil is also manufactured and sold under the name of Tadacip by the Indian pharmaceutical company Cipla in doses of 10 mg and 20 mg.

On November 21, 2003 the FDA approved tadalafil (as Cialis) for sale in the United States as the third ED prescription drug pill (after sildenafil citrate (Viagra) and vardenafil (Levitra)). Cialis's 36-hour effectiveness earned it the nickname, "The Weekend Pill";[1] like sildenafil and vardenafil, tadalafil is recommended as an 'as needed' medication. Cialis is the only one of the three that is also offered as a once-daily medication.

Moreover, tadalafil was approved in May 2009 in the United States for the treatment of pulmonary arterial hypertension and is under regulatory review in other regions for this condition. In late November 2008, Eli Lilly sold the exclusive rights to commercialize tadalafil for pulmonary arterial hypertension in the United States to United Therapeutics for an upfront payment of $150 million.

Source: http://en.wikipedia.org/wiki/Tadalafil

And finally wider info on erectile dysfunction:

Erectile dysfunction

Erectile dysfunction (ED) is sexual dysfunction characterized by the inability to develop or maintain an erection of the penis during sexual performance.[1]

A penile erection is the hydraulic effect of blood entering and being retained in sponge-like bodies within the penis. The process is often initiated as a result of sexual arousal, when signals are transmitted from the brain to nerves in the penis. The most important organic causes are cardiovascular disease and diabetes, neurological problems (for example, trauma from prostatectomy surgery), hormonal insufficiencies (hypogonadism) and drug side effects.

Psychological impotence is where erection or penetration fails due to thoughts or feelings (psychological reasons) rather than physical impossibility; this is somewhat less frequent but often can be helped. Notably in psychological impotence, there is a strong response to placebo treatment. Erectile dysfunction, tied closely as it is about ideas of physical well being, can have severe psychological consequences.

Besides treating the underlying causes such as potassium deficiency or arsenic contamination of drinking water, the first line treatment of erectile dysfunction consists of a trial of PDE5 inhibitor drugs (the first of which was sildenafil or Viagra). In some cases, treatment can involve prostaglandin tablets in the urethra, injections into the penis, a penile prosthesis, a penis pump or vascular reconstructive surgery.[2]

The Latin term impotentia coeundi describes simple inability to insert the penis into the vagina. It is now mostly replaced by more precise terms.[vague] The study of erectile dysfunction within medicine is covered by andrology, a sub-field within urology.

Source: http://en.wikipedia.org/wiki/Erectile_dysfunction