D Introduction The term "opiate" refers to any compound which is derived from the opium plant or poppy. An opiate is by definition a naturally occurring substance. The more inclusive term, opioid, refers to all naturally occurring opiates as well as those molecularly similar compounds which are synthetically derived. Unlike some substances of abuse, such as alcohol and cocaine, which act more generally upon the central nervous system, opioids act through specific receptors. There are naturally occurring opioid receptors in all mammalian brains. Although there are several different subtypes of receptors (such as kappa and delta), the most prominent and the one most responsible for the physiological and psychological consequences of opioid ingestion, including analgesia, is the (mu) JL-receptor. All known opioids exert their most important actions principally through the JL-receptor.