An opioidergic agent (or drug) is a chemical which functions to directly modulate the opioid neuropeptide systems (i.e., endorphin, enkephalin, dynorphin, nociceptin) in the body or brain. Examples include opioid analgesics such as morphine and opioid antagonists such as naloxone. Opioidergics also comprise allosteric modulators and enzyme affecting agents like enkephalinase inhibitors.