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. They’re often the first line of treatment for trouble getting or keeping an erection, called erectile dysfunction ED. Patients should always ask their doctors for medical advice about adverse events. You can take sildenafil with or without food. Take sildenafil as needed before sexual activity. Before taking sildenafil, tell your doctor and pharmacist if you are allergic to sildenafil, any other medications, or any of the ingredients in sildenafil tablet or suspension. Sildenafil does not prevent pregnancy or the spread of sexually transmitted diseases such as human immunodeficiency virus HIV.According to price-comparison site , generic Revatio will cost you as little as $ per pill, depending on where you fill your prescription. That’s compared with . Overdose Ask your doctor Q&A Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription . You should also avoid products you can buy online without a prescription that claim either to be generic forms of sildenafil or to contain sildenafil. (Sildenafil is the active drug .