Fig. (1) Classification of opioids. Classification for this review was based on receptor effect and receptor type. Classification according to effect is three types, agonist (when a particular chemical binds to its receptor and activates it), the antagonist (a chemical that binds to the receptor blocking it and thereby dampening any downstream effect) and partial agonist (have similar mechanisms as an agonist but only produce a partial effect). The classification according to the receptor type is Mu, Delta, Kappa and the more recently discovered Nociceptin. The effect produced by each type expect for Nociceptin is seen in the figure above.