Illustration of the molecular structure of fentanyl (C22.H28.N2.O), a synthetic opioid used to alleviate severe pain resulting from surgery, injury, or illness. Fentanyl works by preventing pain signals from reaching the brain. It is very potent, 50 to 100 times stronger than morphine, and can be highly addictive. Fentanyl is often linked to fatal and nonfatal overdoses. Atoms are represented as spheres and are colour-coded: carbon (grey), hydrogen (white), nitrogen (blue) and oxygen (red).