Viagra (sildenafil) is an anti-impotence drug specially designed for the needs of males willing to carry on with their sex life but unable to get an erection hard enough to have sex. Before taking Viagra, tell your doctor if you are allergic to any drugs, or if you have bleeding disorder, liver disease, blood cell disorder, stomach ulcer, kidney disease, heart disease, high or low blood pressure, congestive heart failure, or heart rhythm problems, as these conditions may be important for determining the right dose for you. Viagra must never be combined with nitrate-based drugs often used for the treatment of chest pain. Nitrate based drugs are prescribed quite often, so you need to be sure any meds you are taking at the moment are not classified as nitrate-based meds. A combination like that can lead to a sudden drop in blood pressure and cause fainting, stroke and a heart attack. Unlike commonly believed, Viagra does not give an instant erection. Once you have taken it, Viagra becomes efficient after 40-50 minutes, but certain amount of sexual stimulation is always required for you to get an erection. It will provide you with up to 4 hours of efficiency, although it’s best to attempt having sex right after it becomes efficient ( 40-50 minutes after you have taken the dose).