Opioids trigger the release of endorphins into the body, triggering a sense of euphoria and reduction of pain. The longer a person takes opioids, the fewer endorphins the body releases as a response to the same dose. This often prompts the person to take more or larger doses to get the same sensation.
If the person chooses not to take a higher dose, or stops taking the drug altogether, it can trigger withdrawal symptoms – which include increased pain, cold sensations, anxiety, fatigue, and diarrhea.