What should I do if I forget a dose? Take sildenafil at around the same times every day. If you are taking sildenafil to treat erectile dysfunction, follow your doctor’s directions and the guidelines in this paragraph. It might work longer if you have mild to moderate erectile dysfunction. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. However, if it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule.Before having surgery, tell your doctor or dentist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products). During pregnancy, . Sildenafil tablets for erectile dysfunction come in different strengths ranging from 25mg to mg. The usual dose is 50mg, when you need it. Do not take it more than once a day. The dose can be increased to mg or decreased to 25mg depending on its effect. Take sildenafil up to 4 hours before you want to have sex.  · Below are details about Viagra dosages commonly used to treat erectile dysfunction (ED).. Viagra form. Viagra comes as tablets that you swallow. Viagra strengths. Viagra tablets are available in.