Drugs in general have what is called a
half-
life in metabolism to active form, and also a
half-
life for the extraction by the kidneys of the active form. A half-life is simply the time taken to remove/destroy one half of the drug. If T = 3hrs is the half-life to eliminate the active form of a particular drug, then once T hrs have elapsed you have only 50% of the active form left. Once another T hours have elapsed, ie 6hrs, there will be only 25% of the active form left.