Contain the wrong ingredients or other harmful substances. The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 minutes before sexual activity. It works for up to 36 hours. Some examples of nitrates are nitroglycerin Nitro-Dur, Nitrostat, others , isosorbide mononitrate Monoket and isosorbide dinitrate Bidil. An erection results from an increase in blood flow to the penis.Published June 25, Because it improves blood flow to the penis, Viagra (sildenafil citrate) treats erectile dysfunction (ED) effectively. But now that the “little blue pill” is . Have an accurate diagnosis and prescription: Remember that Viagra and generic sildenafil are not available over the counter — you will need a prescription. Some . Brand names Why is this medication prescribed? Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men.