because fentanyl is so much more powerful that heroin it has to be cut with some inert substance. the problem is that if the fentanyl isn't mixed well there can be "hot spots" in the mix that are 100x more potent than the rest. the tools that drug dealers use to cut the fentanyl are not exactly known for lab quality precision, nor are the people using them. the lure of fentanyl for the dealers is that being 10 to 20 times stronger than heroin, they only have to smuggle one kilo compared to ten to twenty kilos for heroin.   nalgesic that's the whole problem with fentanyl.
United States