Its not as bad as directly stealing money from someone, but it is stealing. If you are scraping by and you dont have the money for shit they are ruining you.
The problem is that getting on worse terms with your roommates can end up badly. I spent two years with some roommates that resulted in a falling out. It really sucked. arguments over who left the dishes, who bought a new router without asking the others from the shared monthly house-wallet, who eats all the fucking chickpeas, someone drinks too much milk, who is stinking up the bathroom, why did you call the landlord.
All this type of shit just adds up, but if you're polluting your food it might begin a shit storm of him taking revenge. If his personality is to lie to you, and you poison his food with laxatives, you might embarass him for life. he might literally be in a public place and shit his pants. it could work out as the perfect karma-bitch lesson that everyone here including myself think he deserves.
But if he is the revenge type, it might open up a can of worms. one that ends with him ruining your shit, poisoning your food, going into your room and doing shit that you can prove was him. it can become toxic to be in your home, and it will be warfare. Someone else on GAF had a story about a roommate who was a shithead and their family came and trashed his room before they left. ruined computer, tore up his clothes with a scissor.
being a student, without insurance, without proof. you can get in over your head, and sometimes its worth it, but here is what I would do.
I would go to a second hand store or look for a small mini fridge to have in your room. put your meals in there. a mini fridge is not expensive nor is it big. the biggest problem is that the cheap ones make a lot of noise. its good for storing cold beverages and food for several days.
I wish you could do the laxatives. he deserves it. it really is a mean thing to do. to steal from you. I would take it personal. Perhaps too personal. Maybe that is why I think the mini fridge is a better idea. If you are the impulsive sort or he is, this shit will escalate. I wouldn't let this go. But I am also poor enough for it to matter if someone took my food. I would literally starve so I would consider it the consequences as abusive and destructive for my being.