This increased blood flow can cause an erection. Do not stop taking sildenafil without talking to your doctor. Some examples of nitrates are nitroglycerin Nitro-Dur, Nitrostat, others , isosorbide mononitrate Monoket and isosorbide dinitrate Bidil. This can happen if you are sick with fever, diarrhea, or vomiting; sweat a lot; or do not drink enough liquids. If you are taking sildenafil for erectile dysfunction, your doctor will probably start you on an average dose of sildenafil and increase or decrease your dose depending on your response to the medication. If you ever need emergency medical treatment for a heart problem, the healthcare providers who treat you will need to know when you last took sildenafil. This medication is taken with or without food an hour before sex.Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or alcohol. Its . Generic Viagra (sildenafil) is available as a , , , and milligram (mg) pill and in liquid form. It’s meant to be taken as needed — about an hour before sex — but . mg Typical dosages The following information describes commonly recommended dosages for ED. However, be sure to take the dosage your doctor prescribes for you. .