Sildenafil is a drug used for erectile dysfunction in men. It is the generic form of the well-known drug, Viagra. The patent for Viagra expired in 2013, which allowed manufacturers other than Pfizer to make the medication. This, in turn, drove down the price of the active ingredient, Sildenafil. Sildenafil is a popular choice of treatment for erectile dysfunction, as medically there is no difference between Sildenafil and its branded alternative, yet Sildenafil is a much more affordable option.
You can also read more about Sildenafil, other ED treatments and condition information at the NHS Choices website.