"Extensive metabolizers," who make up most of the population, break down 10 to 200 times more of the drug in a given time period than do the other group.

Women who are extensive metabolizers of tamoxifen related to CYP2D6 may have more severe hot flashes than those who are poor metabolizers.

Drug levels can be as much as fourfold greater in persons who are poor metabolizers than in persons who are homozygous extensive metabolizers.

For Razadyne ER (the once-a-day formulation), CYP2D6 poor metabolizers had drug exposures that were approximately 50% higher than for extensive metabolizers.

Conversely, 0.5-2% of the population are "extensive metabolizers"; multiple copies of the gene for 2D6 produce high levels of CYP2D6 and will metabolize drugs through that pathway more quickly than others.

Extensive metabolizers (that is, people who are extensive metabolizers of a given type) are the most common, and are the type of people for which drugs are designed.

However, extensive and poor cytochrome 450 CYP2D6 metabolizers had similar physiological and subjective responses to hydrocodone, and CYP2D6 inhibitor quinidine did not change the responses of extensive metabolizers, suggesting that inhibition of CYP2D6 metabolism of hydrocodone has no practical importance.