For the most part it's not intentionally lacing drugs, it's poor quality control and cross contamination.
No one is intentionally mixing fentanyl and MDMA or cocaine but with poor lab work it's easy to get a bit of the wrong whitish powder and the people selling it down stream have no more idea than their buyers what's really in it.
Affluent recreational drug connoisseurs can use basic chemistry and test kits to be highly confident what they're taking but even they get bit sometimes if reckless and using a new source.
No one is intentionally mixing fentanyl and MDMA or cocaine but with poor lab work it's easy to get a bit of the wrong whitish powder and the people selling it down stream have no more idea than their buyers what's really in it.
Affluent recreational drug connoisseurs can use basic chemistry and test kits to be highly confident what they're taking but even they get bit sometimes if reckless and using a new source.