Connect with one of Felix’s healthcare practitioners today. If you feel nervous about how to talk to your doctor about ED, try using this checklist to help you find the right words. 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. If you are taking sildenafil for erectile dysfunction, you are unlikely to miss a dose since this medication is taken as needed, not on a regular dosing schedule. A newer form of the medicine that dissolves on the tongue might work faster than the pill that you swallow. Anticonvulsants like phenytoin or carbamazepine and an antibiotic called rifampin can all cause Viagra to not do its job.Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). The drug comes as a tablet you’ll take by swallowing. It’s usually taken as needed about 1 . Generic Viagra (sildenafil) is an alternative to the branded Viagra drug that can be used for treating erectile dysfunction (ED). It’s now sold by many manufacturers, including online. Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription drug that’s used in adult males* .