A class of analgesics structurally similar to the natural alkaloids derived from the resin of the opium poppy. Natural alkaloids are known as opiates and they include morphine and other similarly structured drugs, such as codeine, hydrocodone, and oxycodone. Synthetic derivates include hydromorphone, fentanyl, and heroin, among others (3). Three opiate receptors – µ, ?, and d – were found predominantly in the CNS. Most analgesic effect of opioids is mediated by the µ receptor. Decrease presynaptic calcium influx and increase postsynaptic potassium efflux, leading to inhibition of neuronal firing and neurotransmitter release.