From Wikipedia:

Sildenafil is available as a generic drug in the United States.[55] As of 2016 branded pills cost about 50 times more than generic ones.[55]

In the United States, Pfizer received two patents for sildenafil: one for its indication to treat cardiovascular disease (marketed as Revatio) and another for its indication to treat erectile dysfunction (marketed as Viagra). The substance is the same under both trade names.[55]

In 1992, Pfizer filed a patent covering the substance sildenafil and its use to treat cardiovascular diseases.[56] This would be marketed as Revatio. The patent was published in 1993 and expired in 2012. The patent on Revatio (indicated for pulmonary arterial hypertension rather than erectile dysfunction) expired in late 2012. Generic versions of this low-dose form of sildenafil have been available in the U.S. from a number of manufacturers, including Greenstone, Mylan, and Watson, since early 2013.[57] Health care providers may prescribe generic sildenafil for erectile dysfunction.[55] However, the generic is not available in the same dosages as branded Viagra, so using dosages typically required for treating ED requires patients to take multiple pills.[55]

In 1994, Pfizer filed a patent covering the use of sildenafil to treat erectile dysfunction.[58] This would be marketed as Viagra. This patent was published in 2002 and will expire in 2019. Teva sued to have the latter patent invalidated, but Pfizer prevailed in an August 2011 federal district court case.[59] In an agreement with Pfizer, Teva will begin to provide the generic drug in 2017.[5

Phosphodiesterase Inhibitors Onset, Duration, Side Effects, etc.

http://emedicine.medscape.com/article/444220-medication#

Generic viagra is readily available from Canadian pharmacies on-line (plus a lot of other stuff0