Viagra (sildenafil) is a drug manufactured by Pfizer company that became the first medicine to be approved by the FDA for the treatment of impotence. This disease had been overlooked for years before that, and the appearance of a single blue pill that could be taken as needed revolutionized the approach to erectile dysfunction (impotence) and its treatment. Viagra is a pill that you can take just as needed, about an hour before the planned sexual activity. Once taken, this medicine will provide you with up to 5 hours of efficiency - this means that you can have sex any moment within this period. However, keep in mind that the period mentioned may be slightly different for different people, so Viagra is most efficient if you have sex about an hour after you have taken it. Viagra comes in three different dosages - containing 25, 50 and 100 mg of the active ingredient. If you have certain health conditions or are taking some medications your dose may be the lowest at first, but your health care provider will probably increase it if you are not benefitting from the dose prescribed and there are no abnormal body reactions to this drug. Do not increase the dosage yourself to make sure there are no side effects.