If there is 50mg of
morphine in the body, 25mg (50%) will be lost
in 3 hours; if there is only 2mg of
morphine in the body, only 1mg (50%) will be lost in 3 hours. Note that in both cases, morphine levels drop by 50% during an interval of one half-life. (3 hours)
If you took a 2mg dose of Xanax, within 12-15 hours you would have lost half, or 1mg. Within the next 12-15 hour window, you would lose 1/2 of THAT amount or .50mg.