Sudden vision loss in one or both eyes can be a sign of a serious eye problem called non-arteritic anterior ischemic optic neuropathy NAION. Some examples of nitrates are nitroglycerin Nitro-Dur, Nitrostat, others , isosorbide mononitrate Monoket and isosorbide dinitrate Bidil. Severe liver disease. These medicines all work well for ED. Medicines that you take by mouth are called oral medicines. Do not take more than one dose of Viagra or Sildenafil daily, or use as directed as often as your doctor has recommended sometimes doctors recommend only a couple of times a week.CHOOSE BRAND-NAME VIAGRA. The original little blue pill prescribed by doctors for over 20 years. Clinically proven to treat men with erectile dysfunction (ED) Ongoing . Drug Images. Viagra treats erectile dysfunction (ED) in adults. It’s usually taken about 1 hour before sexual activity. Below, you’ll find information about recommended dosages . The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 .