Opioids act by attaching to specific proteins called opioid receptors, which are found in the brain, spinal cord, and gastrointestinal tract. When these drugs attach to certain
opioid receptors, they can block the transmission of pain messages to the brain. In
addition, opioids can produce drowsiness, cause constipation, and, depending upon he amount of drug taken, depress respiration. Opioid drugs also can cause euphoria by affecting the brain regions that mediate what we perceive as pleasure.