Your brain on Fentanyl - YouTube   2:14 minute video

Fentanyl activates opioid receptors in the brain leading to the release of dopamine. The release of dopamine is what leads to the euphoric high that people usually experience when taking fentanyl.  However, the brain quickly gets accustomed to the presence of fentanyl and begins to compensate for the influx of dopamine. This can lead to physical dependence and withdrawal.