All drugs that can be misused, from nicotine to heroin, cause a powerful spike of dopamine in the brain, making it hard for the brain to ignore. Although opioids can affect people differently, most people who become addicted to opioids report feelings of euphoria and relaxation. Over time, this ‘high,’ combined with the surge in dopamine that tells the brain to continue repeating the behavior, rewires the reward system of the brain so that a person will continue to crave, seek out and use an opioid despite negative consequences.
As a person uses opioids with more frequency, their body will become so used to the substance that they actually have to take more of the drug to get the same effects. This is called tolerance. As tolerance builds, it becomes harder and harder to stop using a drug because a decrease in use will lead to symptoms known as withdrawals. In the case of opioid use disorder, withdrawals are often described as feeling like the worst flu one could imagine, including vomiting, diarrhea, muscle aches, abdominal cramping, excessive sweating, racing heartbeat, high blood pressure, runny nose, and inability to sleep. For most people who experience a severe opioid use disorder, they reach a point of opioid use in which they no longer experience euphoria when they use; they are simply using opioids to avoid feeling sick from withdrawal symptoms.