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Brain
Multiple regions in the brain are home to opioid receptors. Areas that regulate pain perception and emotional reward are those most affected by the drug, creating senses of both euphoria and pain relief at the same time. Receptors in the brain are susceptible to Pavlovian conditioning; that is, desire for relief and euphoria grows as more of a drug is taken.
Brain Stem
The brain stem is particularly vulnerable to an overdose. Cardiac and respiratory control centers are found in the stem; when receptors are flooded with opioids, breathing and heart rates drop. Some deaths occur within minutes, but most often there is a period of inactivity lasting several hours after opioid intake.
Spinal Cord
A dense cluster of receptors resides within the dorsal horn of the spinal cord, which is the intended target of prescription opioids. By binding to these receptors, opioids reduce pain signals that originate from sickness, injury or surgery.
Intestines
An uncomfortable side effect of opioid medication derives from receptors in the intestinal tract. When the receptors are activated, peristalsis—the mechanism of moving food through the body—stops. A blockage then forms in the tract, hence the recent prevalence of medication that combats opioid-induced constipation.