Sildenafil Citrate (Viagra) belongs to a group of medicines that delay the enzymes called phosphodiesterases from working too quickly. The penis is one of the areas where these enzymes work. Generic Viagra Sildenafil is used to treat men who have erectile dysfunction (also called sexual impotence). By controlling the enzyme phosphodiesterase, Generic Viagra sildenafil helps to maintain an erection that is produced when the penis is stroked.