Metabolism of opioids into metabolites that can cross the blood-brain barrier is mediated by CYP-450 series enzymes in the human liver. Because of natural genetic polymorphism between individuals, (1) some people are “poor” metabolizers, (2) some are “average” metabolizers, and (3) some are “hyper” metabolizers.
An estimated 20-30 percent of pain patients fall into category 1 or category 3 above. To provide adequate pain control, poor opioid metabolizers may need many times the opioid dose levels recommended by FDA for average metabolizers. The same is also true for “hyper” metabolizers.