Sildenafil under the brand name Viagra is used to treat erectile dysfunction. Sildenafil under the brand name Revatio is used to treat pulmonary arterial hypertension and improve exercise capacity in both sexes. Do not take both brands at once. Sildenafil relaxes muscles in blood vessels, increasing blood flow to areas of the body such as the penis. Do not give this medication to patients under 18 without medical approval.

Generally, Revatio is taken 3 times a day every 4-6 hours. Viagra is taken as-needed, 30 minutes to 1 hour before sexual activity, and up to 4 hours before sexual activity. Do not take Viagra more than once a day. 

Do not take sildenafil if you also take a nitrate drug, including recreational “poppers.” Alert your doctor to all other drugs you may be taking, as sildenafil can interact dangerously with them, especially riociguat, antifungals, HIV/AIDS medicine, prostate disorder medicine, and similar medicines like tadalafil. Tell your doctor your complete medical history prior to taking a new drug, especially if you smoke, if you’ve had heart or circulation problems, vision problems, blood cell disorders, or have been told not to have intercourse for other health reasons. Tell your doctor if you become pregnant or breast-feed. Avoid grapefruit products and other medications that treat impotence.

Find immediate emergency assistance if you experience serious side effects such as: allergic reaction, overdose, erection that lasts for more than 4 hours, painful erection, sudden vision loss, hearing problems, heart attack symptoms, convulsions, shortness of breath, and lightheadedness. If you notice a serious symptom such as the following during sexual activity, stop and contact your doctor: pain, numbness, dizziness, nausea, tingling. Additional side effects of sildenafil include: flushing, headache, abnormal vision, congestion, nosebleeds, insomnia, muscle or back pain.