Viagra (Sildenafil) is used to treat male sexual function problems (erection dysfunction). Duration of action of this drug is four hours and better be taken one-half hour before sexual activity. Viagra works along with sexual stimulation to help achieve an erection. 

Common side effects include headache, flushing, stomach upset or nausea, heartburn, stuffy or runny nose, diarrhea, dizziness, or lightheadedness. Do not take Levitra if you are using a nitrate drug  (Nitrostat, Nitrolingual, Nitro-Dur, Nitro-Bid), isosorbide dinitrate (Dilatrate-SR, Isordil, Sorbitrate), and isosorbide mononitrate (Imdur, ISMO) for chest pain or heart problems. Taking Levitra with a nitrate medicine can cause a serious decrease in blood pressure, leading to fainting, stroke, or heart attack.