But medication may be just one of many ways to help you stay hard. Kidney disease that needs to be treated with dialysis. Anticonvulsants like phenytoin or carbamazepine and an antibiotic called rifampin can all cause Viagra to not do its job. Always take Viagra under the supervision of a licensed medical professional, who can properly monitor your dose and any side effects. Tell your doctor if sildenafil is not working well or if you are experiencing side effects.Take sildenafil as needed before sexual activity. 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 . 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* . Never order medicines from an online pharmacy that doesn’t list its phone number and physical address, has prices that seem too good to be true, or tells you that no .