One reason for the high mortality rates associated with fentanyl use is its extremely high efficacy and narrow therapeutic index ( LD50 ? ED50 = 277, as opposed to 25,211 for sufentanil ). [18] Fentanyl can be fatal in an adult of normal weight as little as 2 mg. [19] [20] Fentanyl is increasingly used to cut heroin . If the fentanyl is not properly distributed through the heroin, fatal fentanyl hotspots occur. [21] Fentanyl, which is cut or sold directly on the black market, comes partly from legal pharmaceutical and partly from illegal production. [22]

By altering residues in the fentanyl molecule, a range of different designer drugs are synthesized, such as methylfentanyl and benzylfentanyl.