Often, these are prescribed for chest pain called angina. Some medications should not be taken with sildenafil. May make this condition worse. It works for up to 36 hours. Severe liver disease.Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in . Viagra (sildenafil) is a phosphodiesterase-5 (PDE5) inhibitor used for the treatment of erectile dysfunction. What Are Side Effects of Viagra? Side effects of Viagra include: . Viagra is a brand-name prescription drug that’s approved to treat erectile dysfunction (ED). With ED, you’re unable to have or maintain an erection. Viagra is approved for this use .