There have been rising number of deaths in the UK, Europe, and North America which have been linked to the use of two opioids - fentanyl and carfentanil. These drugs are reportedly being mixed with heroin, often without the knowledge of the person making the purchase. (Release, UK, 28.04.2017)

http://www.release.org.uk/blog/harm-reduction-people-who-use-or-may-encounter-fentanylcarfentanil