Viagra (sildenafil citrate) is a highly efficient treatment for erectile dysfunction that has been in the market of ED medicine for over 10 years. Viagra stimulates blood flow into the penis allowing it to expand more than usual and preventing the blood in the tissues to escape back into the body too soon. This gives the patient some time to perform sexually - for most people it's 4 hours but depending on the patient's age and health condition this period of time can be shorter. Your health care provider will most likely start you or the lowest dosage of Viagra to see how you respond. If no benefits are seen the dose may be later adjusted. If you have recently used nitrate-based drugs (isosorbide mononitrate, isosorbide dinitrate, or nitroglycerin) that can come in the form of sprays, tablets, pastes and ointments you must not take Viagra as at least two weeks are supposed to pass after the last dosage of a nitrate-based medicine.