Viagra (sildenafil) is a brand-name prescription drug that’s used to treat erectile dysfunction (ED). Learn about side effects, warnings, dosage, and more. Find out how sildenafil treats erection problems and pulmonary hypertension and how to take it. NHS medicines information on sildenafil – what it's used for, side effects, dosage and who can take it. Viagra (sildenafil) is a prescription-only medication used to help treat erectile dysfunction (ED) in men. Learn more about what Viagra is, including what it’s used for, how it works, side effects, potential drug interactions and the difference between Cialis vs Viagra. What is Viagra? Viagra is a prescription medicine used to treat erectile dysfunction (ED) in men — the inability to achieve or maintain an erection sufficient for sexual activity. Viagra relaxes muscles found in the walls of blood vessels and increases blood flow to particular areas of the body. Viagra is used to treat erectile dysfunction (impotence) in men. Learn more about Viagra, including its uses, dosage, side effects, and how to get it. Viagra is a prescription medicine used to treat the symptoms of erectile dysfunction. Learning about dosages, side effects, drug interactions, warnings, and more.