When opioids enter the brain, they attach to molecules on cells known as opioid receptors. These receptors are located in many areas of the brain and body, especially areas involved in feelings of pain and pleasure, as well as a part of the brain that regulates breathing.
Opioids affect the brain's "reward circuit," causing euphoria (the “high”) and flooding the brain with the chemical messenger dopamine. Surges of dopamine in the reward circuit encourage you to continue pleasurable but unhealthy behaviors like taking drugs, leading you to repeat the behavior again and again. Repeated surges from drug-taking can lead to addiction.
Learn more about how the brain works and what happens when a person misuses drugs. And, check out how the brain responds to natural rewards and to drugs.