Codeine is an old and commonly used analgesic agent for mild to moderate pain. It is the prototypical "prodrug" in that its analgesic effect is almost wholly dependent on its biotransformation to morphine, a process that is mediated by the polymorphic cytochrome P450 2D6 enzyme. As such, interindividual variability in codeine metabolism and response is a clinical reality, and there has been much progress in characterizing the genetic causes of this variability in diverse populations. Yet despite the potential for both life-threatening adverse reactions and lack of therapeutic effect, codeine is not commonly indicated for therapeutic drug monitoring. This review will discuss the relative role of pharmacogenetics and therapeutic drug monitoring in predicting and/or maintaining adequate and safe analgesia with codeine. The review will end on a discussion of how the marriage of these 2 fields may provide new insights into the mechanisms of codeine-induced toxicity and analgesia.