Opioids look like chemicals in your brain and body that attach to tiny parts on nerve cells called opioid receptors. Scientists have found three types of opioid receptors: mu, delta, and kappa (named after letters in the Greek alphabet). Each of these receptors plays a different role. For example, mu receptors are responsible for opioids’ pleasurable effects and their ability to relieve pain.
Opioids act on many places in the brain and nervous system, including: