Oh FFS, Guardian. Can you please hire one person with a science degree before trying to write about health or science topics.
Carfentanil and fentanyl are different substances. They are both synthetic opioids, and the former has about 100 times the potency of the latter. One is not “a type of the other”. This is actually a nice illustration of the confusion that kills people.
Here’s a table comparing the equivalent doses of these drugs:
As you will see, carfentanil’s dosage is invariably almost too small to measure.