Long-term opioid use changes the way nerve cells work in the brain. This happens even to people who take opioids for a long time to treat pain, as prescribed by their doctor. The nerve cells grow used to having opioids around, so that when they are taken away suddenly, the person can have lots of unpleasant feelings and reactions. These are known as withdrawal symptoms.
Have you ever had the flu? You probably had aching, fever, sweating, shaking, or chills. These are similar to withdrawal symptoms, but withdrawal symptoms are much worse.
That is why use of opioids should be carefully watched by a doctor—so that a person knows how much to take and when, as well as how to stop taking them to lessen the chances of withdrawal symptoms. Eventually, the cells will work normally again, but that takes time.
Someone who is addicted to opioids has other problems as well. For example, they keep taking the drug even though it may be having harmful effects on their life and their health. They have strong urges to take the drug—called cravings—and they no longer feel satisfied by natural rewards (like chocolate, TV, or a walk on the beach).