Fentanyl is up to 50 times stronger than heroin and 100 times stronger than morphine. This powerful synthetic opioid is involved in most drug overdose deaths in the U.S. today.
Used to sedate elephants and other large animals, carfentanil is 10,000 times stronger than morphine and 100 times stronger than fentanyl and not intended for human use.
See a list of opioid drugs.