Therefore, doctors recommend taking just one pill of Viagra per day. An inherited eye disease that affects the retina. The blue pill works by inhibiting an enzyme called phosphodiesterase type 5 PDE-5 , making muscles in the penis more likely to relax, and allow blood to flow in, causing an erection. Published by Medicaldaily. Some examples of nitrates are nitroglycerin Nitro-Dur, Nitrostat, others , isosorbide mononitrate Monoket and isosorbide dinitrate Bidil.Oral medicines for ED usually work well and cause few side effects. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil . Medically reviewed by Melissa Badowski, PharmD, MPH, FCCP — By Jennifer Mitri Williamson, Pharm.D. — Updated on September 12, Learn about .