Sildenafil is in a class of medications called phosphodiesterase (PDE) inhibitors. Sildenafil treats erectile dysfunction by increasing blood flow to the penis during sexual stimulation. This increased blood flow can cause an erection. Do not take it more than once a day. The dose can be increased to 100mg or decreased to 25mg depending on its effect. Take sildenafil up to 4 hours before you want to have sex. For sildenafil to work properly, you'll need to be sexually excited.