Tell your doctor or pharmacist if you are taking nitrates medications for chest pain such as isosorbide dinitrate Isordil , isosorbide mononitrate Monoket , and nitroglycerin Nitromist, Nitro-Dur, Nitrostat. 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. Take sildenafil as needed before sexual activity. Be sure to let your doctor and pharmacist know that you are taking these medications before you start taking sildenafil. If you become pregnant while taking sildenafil, call your doctor. Kidney disease that needs to be treated with dialysis. Sildenafil does not prevent pregnancy or the spread of sexually transmitted diseases such as human immunodeficiency virus HIV. · Maximum dose: 20 mg orally three times a day. Injection: Initial dose: or 10 mg IV bolus three times a day. Comments: No greater efficacy was achieved with oral doses higher than the maximum recommended dose. A 10 mg injection dose is predicted to provide pharmacological effect equivalent to the 20 mg oral dose. There is also a generic version of Viagra available, under the name sildenafil, in 25mg, 50mg, and mg strengths. 3. Downsides. If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include. Taking sildenafil with a nitrate medicine can cause a sudden and serious decrease in blood pressure. Contact your doctor or seek emergency medical attention if your erection is painful or lasts longer than 4 hours. A prolonged erection (priapism) can damage the penis. Viagra mg (Pfizer VGR ) View all images.