Fentanyl is frequently being mixed with a potent animal tranquilizer called 

xylazine (known on the street as "tranq" or "tranq dope"), creating a dangerous and often fatal combination. This mixture has been declared an "emerging threat

So yeah it’s out there

Depends on the area and the fuckhead you're dealing with.

I got some weed with some kind of upper in it once. That was me being a stupid teen tho lol.

I knew a guy who's business model was to pick out potheads he thought would have money and interest and sell them a bag dusted with something else. Pot wouldn't hit the same after it and when they come back complaining he'd offer them the coke or meth he was selling instead.

One and only time I ever narked.

DMX was handed blunts laced with crack when he was a teenager, and it became a dependency until the day he died. That's why people do it in most cases: to create dependency.

I'm a secondary source, and validate your statement.