Differences in response to medical products have been observed in racially and ethnically distinct subgroups of the US population. Approved doses and dosing regimens for some of the most widely prescribed drugs have also been shown to be different in different countries (e.g., the United States, the United Kingdom, and Japan). These differences may be attributable to intrinsic ethnic factors (e.g., genetics, metabolism, race), extrinsic ethnic factors associated with environment and culture (e.g., medical practice, diet, use of alcohol, concomitant drug use, regulatory environment), or interactions between these factors.
Just as it is valuable for a doctor to know that a patient's medical history includes familial breast cancer, thus calling for vigilance for early cancer signs, a patient aware of his genotype could inform his doctor that he may be a poor drug metabolizer, which might guide the doctor in determining drug dosage and foreseeing possible adverse drug reactions.
http://www.slate.com/id/2198731/