I wouldn’t say they are poison, but in my experience they often don’t work as well.
One of the best examples is amphetamines. Name brand Adderall and Dexedrine are significantly more potent than the generic alternatives I’ve tried. Maybe this has to do with the binders, or maybe there is less amphetamines in it, I don’t know.
As an anecdote, my dad works as a pharma regulator for the FDA and never takes generic medication. He’s told me that generics are inadequately regulated to ensure equivalency to the original.