the opioid analgesics, of which morphine is the prototype, vary in potency, efficacy, and adverse effects. These drugs produce their analgesic effects by binding to discrete opiate receptors in the peripheral and central nervous systems. In contrast to the nonopioid analgesics, opioid analgesics, at least the opioid agonists, do not appear to have a ceiling effect (i.e., as the dose is escalated on a log scale, the increment in analgesia is linear of point of loss of consciousness).