Hallucinagenic results would be interesting, a poison for the more fantasy based critters really. Though plant extracts ought to have a similar effect. Though from various different films they can be good as well as bad (Batman begins, conquering fear for example). Bad to base it from films, but good ideas none the less.
Hallucinations done well would mess around with a dwarves pathfinding by scrambling how it moves for example, moves forward, but moves left instead and such, lots of falling over from disorientation. That would be one good simulation.
For ones where the landscape is changed (probably to an extremely evil biome where everything looks and feels alot harder to fight than it really is) with shifting masses of plants, trees, natural obstacles to disorient overlaid the original world so that you bump into walls, then can't will yourself to move when really you can, quite hard to simulate for potentially alot of dwarves in fort mode at the same time, though worth it, any critters seen by them would naturally be distorted randomly from friendly ones being assumed evil to benine ones seeming aggressive.
Surviving these hallucinations would naturally build up stats relevant to countering them making it a worthwhile training side, throwing them into an arena with goodness knows what is a plus.
Though some could be really subtle, like all your friends seeming really nasty (or too friendly) and messing up all your relationships. Or inducing a tantrum by messing with happiness and such.