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What is sildenafil? Sildenafil relaxes muscles of the blood vessels and increases blood flow to particular areas of the body. Sildenafil under the name Viagra is used to treat erectile dysfunction (impotence) in men. Another brand of sildenafil is Revatio, which is used to treat pulmonary arterial hypertension and improve exercise capacity in men and women. Warnings Do not take sildenafil if you are also using a nitrate drug for chest pain or heart problems, including nitroglycerin, isosorbide dinitrate, isosorbide mononitrate, and some recreational drugs such as "poppers". Taking sildenafil with a nitrate medicine can cause a sudden and serious decrease in blood pressure. Some medicines can cause unwanted or dangerous effects when used with sildenafil. Tell your doctor about all your current medicines, especially riociguat (Adempas). Do not take sildenafil with riociguat. Contact your doctor or seek emergency medical attention if your erection is painful or lasts longer than 4 hours. A prolonged erection (priapism) can damage the penis. Stop using sildenafil and get emergency medical help if you have sudden vision loss.