The perpetual pursuit of pain elimination has been constant throughout human history and pervades human cultures. In some ways it is as old as medicine itself. Cultures throughout history have practiced the art of pain management through remedies such as oral ingestion of herbs or techniques believed to have special properties. In fact, even Hippocrates wrote about the practice of trepanation, the cutting of holes in the body to release pain. Current therapies for management of pain include the pervasive utilization of opioids, which have an extensive history, spanning centuries. There is general agreement about the appropriateness of opioids for the treatment of acute and cancer pain, but the long-term use of these drugs for treatment of chronic non-malignant pain remains controversial. The pros and cons regarding these issues are beyond the scope of this review. Instead, the purpose of this review will be directed towards the pharmacology of commonly prescribed opioids in the treatment of various chronic pain syndromes. Opium, derived from the Greek word for "juice," is extracted from the latex sap of the opium poppy (Papaverum somniferum). The juice of the poppy is the source of some 20 different alkaloids of opium. These alkaloids of opioids can be divided into 2 chemical classes: phenanthrenes (morphine, codeine, and thebaine) and benzylisoquinolines (agents that do not interact with opioid receptors).