Significant interindividual differences in opioid sensitivity can hamper effective pain treatment and increase the risk for substance abuse. Elucidation of the genetic mechanisms involved in the interindividual differences in opioid sensitivity would help establish personalized pain management. Studies using gene knockout mice have revealed that genes encoding some metabolic enzymes, opioid transporters, and opioid system signal transduction mediators may be candidate genes to predict appropriate kinds and doses of opioids for individuals. Recently, various databases on knockout mice, pharmacogenetics, and gene polymorphisms have been rapidly consolidated. Such information should aid in developing and improving the methods of predicting interindividual differences in opioid sensitivity. In the near future, it will be possible to predict the appropriate kinds and doses of opioids for individuals by analyzing genetic variations contributing to opioid sensitivity.