Clinical implications of opioid pharmacogenetics.

The peer-reviewed literature yields a plethora of examples of variability in patient's responses to medications. The rapidly progressing field of pharmacogenetics offers insight into the variation in responses observed clinically, and in particular for the variability observed among patients administered mu opioid analgesics. Genetic variation leads to interperson variability in drug absorption, distribution, metabolism, and excretion, processes that have an important impact on the observed efficacy and toxicity of a drug. In particular, single-nucleotide polymorphisms (SNPs) in the gene encoding the mu opioid receptor have been linked to the variability in responses to opioids, whereas SNPs within metabolic enzymes that process and eliminate opioids and their metabolites also have an important effect on an individual's response to opioid medications as do SNPs that affect the bioavailability of opioids. In current clinical practice, given the best available evidence, to optimize pain medications each patient is, in effect, given their own analgesic trial. In the near future, pharmacogenetic approaches may be implemented to best predict which medicine from the outset may be most appropriate for an individual-the therapy with the most sustained efficacy and the best side effect profile. In the meantime, pharmacogenetic studies on mu opioid analgesics have provided a molecular foundation supporting opioid rotation in cases in which opioid therapy loses efficacy or becomes associated with intolerable side effects. As more pharmacogenetic research links specific polymorphisms to the pharmacologic effects of specific opioid analgesics, clinicians will continue to improve their understanding of how to prescribe these medications more effectively.