Opioid therapy for chronic pain has been popularized over the past few decades, and a concern has arisen that the analgesic efficacy of opioids is not always maintained over prolonged courses of treatment despite dose escalation and stable pain. Considering the potentially serious adverse effects of opioids, the idea that pain relief could diminish over time may have a significant impact on the decision to embark on this therapy, especially in vulnerable individuals. Possible loss of analgesic efficacy is especially concerning, considering that dependence may make it hard to withdraw opioid therapy even in the face of poor analgesia. This article first reviews the evidence on opioid efficacy when used for the treatment of chronic pain, and concludes that existing evidence suggests that analgesic efficacy, although initially good, is not always sustained during continuous and long-term opioid therapy (months to years). The theoretical basis for loss of analgesic efficacy over time is then examined. Mechanisms for loss of analgesic efficacy proposed are pharmacologic tolerance, opioid-induced hyperalgesia, subtle and intermittent withdrawal, and a number of psychologic factors including loss of the placebo component.