Be sure to let your doctor and pharmacist know that you are taking these medications before you start taking sildenafil. Oral medicines for ED usually work well and cause few side effects. Your doctor will tell you not to take sildenafil if you are taking street drugs containing nitrates. Sexual activity may be a strain on your heart, especially if you have heart disease. Some examples of nitrates are nitroglycerin Nitro-Dur, Nitrostat, others , isosorbide mononitrate Monoket and isosorbide dinitrate Bidil.In combination with sexual stimulation, sildenafil works by increasing blood flow to the penis to help a man get and keep an drug does not protect against sexually . Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in . Sildenafil works on the PDE5 enzyme in the lungs to relax the blood vessels. This will increase the supply of blood to the lungs and reduce the workload of the heart. This .