Why is fentanyl so potent that tiny amounts can kill?

Fentanyl is so dangerous because it is a synthetic opioid described as 50 to 100 times more potent than morphine, which means that a very small quantity has a large effect on the body, and small errors matter. It is also a real medicine: its main use is managing pain in cancer patients and people recovering from surgery.

Where does it come from? Paul Janssen first synthesized it in Belgium at his newly formed Janssen Pharmaceutica in 1959. It has a short duration of action. Like morphine, it works by activating the mu-opioid receptor, and it binds that receptor about as tightly as morphine does. Why it is stronger in the body is unclear, but its high lipid solubility, meaning it dissolves very well in fat, helps it enter the central nervous system. It is highly lipophilic, about 1000 times more than morphine.

Strength becomes a problem outside medicine. Researchers at Los Alamos National Laboratory note that pure fentanyl powder is so strong that small inaccuracies in weight can cause huge variation between batches. Fentanyl is sometimes sold as heroin or oxycodone and has appeared in pills made to look like pharmaceuticals, and such mixing and disguise results in more deaths. In 2006, illegally made fentanyl, often mixed with cocaine or heroin, caused an outbreak of overdose deaths in the United States and Canada, concentrated in Dayton, Chicago, Detroit and Philadelphia.

The numbers followed. From 2011 to 2021, deaths involving prescription opioids stayed stable, while deaths involving synthetic opioids, mainly fentanyl, rose from 2,600 a year to 70,601. As with all opioids, its effects can be reversed with naloxone.