May cause dose-related impairment of color discrimination. Use caution in retinitis pigmentosa (a minority of patients have genetic disorders of retinal phosphodiesterases). Sudden decrease or loss of hearing reported rarely; may be accompanied by tinnitus and dizziness. Hypotension may occur due to vasodilator effects (use with caution in aortic stenosis or hypertrophic obstructive cardiomyopathy). Concurrent use with α-adrenergic antagonist therapy may cause symptomatic hypotension. Rarely, priapism reported (use with caution in conditions which may predispose patients to priapism (sickle cell anemia, multiple myeloma, leukemia)). Vision loss may occur rarely and be a sign of nonarteritic anterior ischemic optic neuropathy (risk may be increased with history of vision loss, low cup-to-disc ratio («crowded disc»), coronary artery disease, diabetes, hypertension, hyperlipidemia, smoking, and >50 years of age). Use not recommended in degenerative retinal disorders (e.g. retinitis pigmentosa). Use with caution in anatomical deformation of the penis (angulation, cavernosal fibrosis, or Peyronie’s disease). Use with caution in bleeding disorders. Use not recommended in hypotension (<90/50 mm Hg), uncontrolled hypertension (>170/100 mm Hg), unstable angina or angina during intercourse, life-threatening arrhythmias, stroke or myocardial infarct within last 6 months, cardiac failure or coronary artery disease causing unstable angina. Use caution in left ventricular outflow obstruction (e.g. aortic stenosis). Not recommended for use in congenital QT prolongation. Use with caution in hepatic impairment (use not recommended in severe hepatic impairment), and in active peptic ulcer disease. Use in end-stage renal disease requiring dialysis not recommended. Use with caution in patients taking α-blockers (may cause hypotension) or medications known to prolong QT interval. Avoid use in patients taking class Ia or III antiarrhythmics. High potential for CYP-mediated interactions (use with caution in patients taking strong CYP3A4 inhibitors). Concomitant use with all forms of nitrates contraindicated. Concurrent use with other treatments for erectile dysfunction not recommended. Use with caution in the elderly.