Sildenafil citrate drug. Polarized light micrograph of crystals of the drug sildenafil citrate, used to treat erectile dysfunction and pulmonary arterial hypertension. Its trade names include Viagra and Revatio. It is a vasoactive drug that inhibits the breakdown of a chemical that regulates blood flow to the penis. As an antihypertensive agent it is used to treat pulmonary arterial hypertension (an increase in blood pressure in the lung blood vessels). Field of view size: 1.485 x 2.227 millimetres.