Heroin is described as a mu-opioid receptor agonist. This means it effectively and rapidly binds to the natural opioid receptors in the brain, and activates them. Normally, our own natural neurotransmitters bind to these receptors in order to control pain, regulate hormones, and keep us feeling good. However, when the mu-opioid receptors are activated by a powerful drug like heroin, a huge amount of dopamine is released. It is this rapid rush of dopamine that creates the feeling of intense euphoria felt by heroin users.