A Study of Visual Effects of Erectile Dysfunction Medications Dosed Daily for Six Months

Inclusion Criteria: * Healthy male subjects or males with mild erectile dysfunction (ED). * Between the ages of 30 and 65 years, inclusive. Exclusion Criteria: * Phosphodiesterase type 5 (PDE5) inhibitor therapy (tadalafil, sildenafil, or vardenafil) within 6 weeks of the start of the study. * Diagnosis of diabetes mellitus. * Any chronic illness or medication that is a risk factor for eye disease or any medication that causes retinal toxicity or affects visual function. * Certain chronic medical conditions including unstable angina pectoris, severe renal \[kidney\] insufficiency, clinically significant hepatobiliary \[liver, bile duct\] disease, cancer, and AIDS/HIV. * A history of clinically significant chronic ophthalmologic disease or any significant visual abnormality identified at the start of the study.