Generic Viagra (Sildenafil) is well-known for over 15 years worldwide. It works within an hour of taking a tablet, and is best taken on an empty stomach. The effects of Viagra will last for four to six hours. Viagra was developed and originally marketed by Pfizer until 2013. When Pfizer’s patent on Viagra expired, the market was open for other companies to produce the medicine under a generic name Sildenafil, rather than the brand name Viagra. Sildenafil contains exactly the same active ingredient as Viagra (sildenafil citrate), and works in the same way. However, because it is the generic version of the medicine, rather than the brand, Sildenafil is considerably cheaper.