It is the length of time it takes for half of the amount of medication taken to leave the body. This is a good estimate for determining how long opiate pain medications stay in your system. Morphine has a half-life of two hours, the shortest for any opiate medication. The longest is mepredrine at three to four hours. After the half-life is reached, the drug will still stay in your system up to three days, but at far less concentrated levels. People taking these medications usually feel the effectiveness begin to wear off soon after the half-life is reached.