As others have said, marijuana is almost never adulterated. At best, you could mix some CBD or non psychoactive hemp to stretch it, but it’d be less potent and weed buyers today often buy for potency. (Not me man, I’m old, gimme that weak dad grass any day!) And of course, everyone has a story about the kid that bought a pound of oregano and thought it was ditch-grass.

For other harder drugs, you do sometimes get adulteration. Fentanyl, being a very powerful opiate, is sometimes mixed in small amounts into other “white powder” drugs, like heroin or cocaine, sometimes along with cheap non-reactive filler like talc. This is to stretch supply and make more money for the dealer. Ideally, this wouldn’t kill their users but:

1. Different drugs act differently, so while it might be “safe” to do a larger heroin dose for a fun occasion, if you dont realize it’s been cut with fentanyl then you can easily overdose by accident. And people already overdose on heroin, the line between a wild time and dead can be rather thin.
2. Drug Dealers aren’t graduate level chemists, often they haven’t graduated anything, and dont realize that some powder they’ve used as a filler is toxic or becomes toxic in combination with other things or when heated, etc.