Viagra (Sildenafil) Tablets 25mg x 4 is a prescription drug that is used to treat erectile dysfunction (impotence). It helps men with erectile dysfunction to achieve and maintain an erection. It is also used to treat pulmonary arterial hypertension and improve exercise capacity in men and women. The drug is taken by mouth and works by helping the blood flow into the penis to maintain sexual stamina and better performance. The active ingredient in Viagra is called Sildenafil. It belongs to a class of drugs known as PDE5 inhibitors. Other brands of Sildenafil are Revatio and Spedra.