Heroin and/or its metabolites—substances the body produces as it processes drugs—bind to and activate specific receptors in the brain called mu-opioid receptors (MORs). Our bodies contain naturally occurring chemicals called neurotransmitters that bind to these receptors throughout the brain and body to regulate pain, hormone release, and feelings of well-being.9 When MORs are activated in the reward center of the brain, they stimulate the release of the neurotransmitter dopamine, causing a reinforcement of drug taking behavior.10 The consequences of activating opioid receptors with externally administered opioids, versus naturally occurring chemicals within our bodies, depend on a variety of factors: how much of a drug is used, where in the brain or rest of the body they bind, how rapidly they get there, how strong and persistent is that interaction it binds, how strongly it binds and for how long, how quickly it gets there, and what happens afterward.
