This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Opium"

Opium resin contains two groups of alkaloids: phenanthrenes (including morphine and codeine) and benzylisoquinolines (including papaverine). Morphine is by far the most prevalent and important alkaloid in opium, consisting of 10%-16% of the total. It binds to and activates μ-opioid receptors in the brain, spinal cord, stomach and intestine. Regular use, even for a few days, invariably leads to physical tolerance and dependence. Various degrees of psychological addiction can occur, though this is relatively rare when opioids are used for treatment of pain, rather than for euphoric effects. These mechanisms result from changes in nervous system receptors in response to the drug. In response to the drug, the brain creates new receptors for opiates. These receptors are "pseudo" receptors and do not work. When the opiates are out of the body, the brain has more receptors than before the use of the drug, but only the same amount of endogenous opiate (endorphins) to fill these receptors.