I had the same idea as >>9 minus the illegality. Besides, there isn't much fun to be had from "dude, I was tripping!" There are way funner, safer, and more legal things you could try. For starters:
But you know something? The #1 thing you need to do is talk to him about this. He may be many things -- immature, a jerk, this and that -- but he's your roommate and a fellow human being. He at least deserves to know what it is that he's doing that upsets you and the extent to which it upsets you. The things above play out like a Home Alone comedy in Anons' minds but the truth of the matter is that they can backfire on you in several ways: for starters, even if he loses, he can make your life a living hell if he tries to pursue matters legally and the judge agrees to hear the case. Then you've also gotta consider, what if Einstein over there decides to one-up you in this "roommate Cold War" by putting serious poison or other dangerous shit in your food and toiletries? Some people lack common sense and some people can be very evil. If you're roommate has either of those attributes then this cute little revenge game really may not be such a great idea after all.
And I've got to repeat, never ever do anything illegal (e.g. LSD). Not because I'm the Moral Police but because if your douchebag roommate figures out what it is that you've done then he can successfully press charges against you. It becomes way harder, if not impossible, for him to prove he has a case if as in the shampoo example he says, "But Your Honor! I know my roommate did this just to dye my hair red!"