Sexual activity may be a strain on your heart, especially if you have heart disease. If you are taking sildenafil to treat erectile dysfunction, follow your doctor’s directions and the guidelines in this paragraph. Its effects might last longer if you have mild to moderate ED. You can take this medicine with or without food 30 minutes before sex, depending on the dose. Some examples of nitrates are nitroglycerin Nitro-Dur, Nitrostat, others , isosorbide mononitrate Monoket and isosorbide dinitrate Bidil.Sildenafil is used to treat male sexual function problems (impotence or erectile dysfunction-ED). In combination with sexual stimulation, sildenafil works by increasing . Why is this medication prescribed? Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Common Viagra side effects may include: flushing (warmth, redness, or tingly feeling); headache, dizziness; abnormal vision (blurred vision, changes in color vision) runny or .