Fentanyl. Harm reduction advice for people who use or may encounter fentanyl or carfentanil

Fentanyl and carfentanil are highly potent, rapidly acting opioid drugs. 

Fentanyl is estimated to be 100 times more potent than morphine, while carfentanil is estimated to be 10,000 times more potent than morphine. As a point of reference, heroin is 4-5 times more potent than morphine.
It is extremely hard, if not impossible, to tell from sight whether heroin has been mixed with fentanyl or carfentanil. If you suspect that your heroin purchase could contain fentanyl or carfentanil, there are some precautions you can take to reduce the potential harms or risk of overdose.
However - both fentanyl and carfentanil are potent at such low doses that there is a significant risk of death if those drugs are being consumed, even if harm reduction steps are taken. (CGL – Change-Grow-Live / Release, UK, 2018)

https://www.changegrowlive.org/sites/cri.org.uk/files/fentanyl_info_leaflet.pdf