How does codeine work?
Codeine is routinely converted to morphine in the body in order for it to be an effective painkiller. The metabolism of codeine to morphine takes place in the liver through the actions of an enzyme called CYP2D6. Most people have normal CYP2D6 activity and their response to codeine is as expected. However, a substantial minority of people— differing by national origin and race (see below)—have CYP2D6 activity that is higher or lower than normal, potentially resulting in excessive (higher activity) or inadequate (lower activity) response to codeine. What genetic factors are at work? The effect of genetics on the CYP2D6 enzyme has been extensively studied, and we now know that total CYP2D6 deficiency occurs in about 6 to 10 percent of Caucasians, 3 to 6 percent of Mexican Americans, 2 to 5 percent of African Americans and about 1 percent of Asians Americans. People deficient in CYP2D6, called “poor metabolizers” (PMs), have inherited a nonfunctional gene from each parent, and have no CYP
