Carfentanil is one of several analogues of the synthetic opioid fentanyl. These compounds have been used as tranquilizers in veterinary medicine. Carfentanil is 10,000 times more potent than morphine and 5,000 times stronger than fentanyl itself. The drug is so dangerous it has been compared to a nerve gas, and some have referred to it as a chemical weapon. Numerous overdose deaths have been attributed to heroin contaminated with carfentanil. Atoms are represented as spheres and are colour-coded: carbon (black), hydrogen (white), nitrogen (blue) and oxygen (red).