Opioids work by acting on opioid receptors, which are found throughout the body. They attach to these receptors like a lock and key, and once attached, they release a slew of dopamine, which blocks pain. This is great for areas of inflammation, but not so great for healthy tissues. When opioids attach to receptors in the brain, they can interact with the nerves that control a person's respiratory rate, slowing it down to dangerously low levels. In the gastrointestinal tract, they cause constipation. Further, the dopamine release in the brain causes a euphoric and calming effect, which can be highly addictive, causing people to desire the drug after it is no longer needed.