Erectile dysfunction is an inability to sustain a firm erection. The following conditions may increase the likelihood of developing erectile dysfunction: drinking alcohol and smoking, fatigue, brain or spinal-cord injuries, liver or kidney failure, multiple sclerosis, Parkinson's disease, radiation therapy (testicles), a history of a stroke, as well as some types of surgery (prostate or bladder). This condition is often treated using impotence drugs. Such drugs can be taken orally, injected into the penis or inserted into the urethra. Viagra was the first erectile dysfunction treatment medication approved by the Food and Drug Administration. Soon Levitra and Cialis followed. All the three drugs mentioned are the most popular and efficient medicines intended for the treatment of erectile dysfunction. Depending on which drug you have chosen you are supposed to take it from 30 to 60 minutes before the planned sexual activity. Viagra, Levitra and Cialis work by relaxing the smooth muscles in the penis when certain sexual stimulation is provided and increase the blood flow. The blood is unable to escape back into the body so soon and the patient can have sex. Unlike commonly thought, taking these drugs does not automatically trigger a reaction.