Not only heroin or oxy, nor even opiates. It can come from cross-contamination, a seller might cut their heroin stash with fentanyl, let's say they use a scale

Not only heroin or oxy, nor even opiates. It can come from cross-contamination, a seller might cut their heroin stash with fentanyl, let's say they use a scale to try to dose it, they don't clean the scale properly and then use it to weight some cocaine. Done, you have cocaine mixed with fentanyl.