Opioid analgesics are the most commonly used drugs in the treatment of moderate to severe pain, both in the acute (e.g., post-operative) and chronic setting (e.g., treatment of cancer pain). All clinicians involved in prescribing potent opioids for pain relief are aware of the large variable effects of these drugs in the delivery of adequate analgesia, as well as in the development of side effects among individual patients. Both animal and human studies indicate that the variability has multiple origins, ranging from biological (such as related to differences in sex, age, ethnicity and genetic make-up), psychological (such as related to individual differences in coping strategies, social and environmental status, expectation/history and placebo response), co-morbidity and nutritional health status (such as related to individual differences in immune and stress response and antioxidant status) and co-medication (with possible influences on metabolism or related to drug-drug interactions) [l]. While some of these factors are known at the time of treatment (e.g., sex, age and co-medication), others are unknown or require additional testing to become apparent (e.g., the genotyping of genes involved in pain and opioid efficacy). To what extent the individual factors
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