This can happen if you are sick with fever, diarrhea, or vomiting; sweat a lot; or do not drink enough liquids. Do not take more or less of it or take it more often than prescribed by your doctor. Sildenafil does not prevent pregnancy or the spread of sexually transmitted diseases such as human immunodeficiency virus HIV. This medication is taken with or without food an hour before sex. 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. Oral medicine for ED also might not be safe or work well if you have: Very low blood pressure, called hypotension, or uncontrolled high blood pressure, called hypertension. Tell your doctor if any of these symptoms are severe or do not go away: headache.Viagra is used to treat erectile dysfunction (ED). This is unlike Revatio, which is used to treat pulmonary arterial hypertension. (This is a condition that causes . AdSay Goodbye to Performance Anxiety and Hello to Improved Sexual Stamina! Reviews and Comparisons Of The Best Male Enhancement Products. See Which Products & Unbiased Reviews · Top Brands Voted By Men · Over Products Tested. 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 .