A dose of the drug is about 100 times more potent than the same amount of morphine.
Both fentanyl and morphine act on the same receptors in the brain as a sedative, to suppress pain and to slow down breathing. Like morphine, fentanyl is used to treat severe pain, sometimes during recovery from surgery. So what makes fentanyl so much worse?
It was designed in 1960 explicitly to act faster and be stronger than morphine.
This is thanks to its chemical structure. While morphine is hydrophilic, meaning it dissolves in water, fentanyl is lipophilic, meaning it dissolves in lipids and fats.
The blood-brain barrier naturally keeps out most hydrophilic molecules, but it lets in many lipophilic ones.
Fentanyl is more than 130 times more likely to enter the brain than morphine — so a similar dose can have a much more significant impact.