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.