It lasts 4 to 5 hours, or longer if you have mild to moderate ED. What side effects can this medication cause? Tell your doctor or pharmacist if you are taking nitrates medications for chest pain such as isosorbide dinitrate Isordil , isosorbide mononitrate Monoket , and nitroglycerin Nitromist, Nitro-Dur, Nitrostat. Your doctor may adjust your dose as needed and tolerated. For treatment of pulmonary arterial hypertension: For oral dosage forms suspension or tablets : Adults—20 milligrams mg 3 times a day. Oral medicines for ED usually work well and cause few side effects.Examples of Viagra’s more commonly reported side effects include: headache. flushing (temporary warmth, redness, or deepening of skin color) stuffy nose. indigestion (upset stomach) mild vision. Tadalafil (Cialis). This medication is taken with or without food an hour before sex. It works for up to 36 hours. You can take it in a small dose daily or in a larger dose as needed. . Why use it? Price Lasts for Pros Cons; Viagra (sildenafil) starts working in 30–60 minutes: depends on insurance, location, and pharmacy: 4 hours: different doses available.