Police say the man told officers he bought the substance earlier that day and that "it was nasty" when he smoked it.
Uniontown Detective Donald Gmitter says a field test determined the substance wasn't pot at all, but police didn't say what it was.
Even though the 21-year-old man didn't technically buy marijuana, he isn't off the hook.
Sgt. Wayne Brown tells the Pittsburgh Tribune-Review that Wednesday's incident remains under investigation, and the man could face a charge of possessing a counterfeit controlled substance.