Using opioids like codeine, fentanyl, heroin, and morphine can change how your brain works. Opioids make your brain release endorphins, which make you feel good and reduce pain. These good feelings are strong but don't last long. Your body and mind can quickly get used to having opioids, leading to dependence. When the opioids wear off, your brain wants those good feelings back quickly. This can lead to opioid use disorder, where opioids are needed just to feel “normal.” Unfortunately, those who want to stop using can find themselves experiencing painful withdrawal symptoms.