The following link is from a tumblr blog that showed a note from one co-worker to another coworker who keeps stealing his or her food from the fridge.
http://vesperblade.tumblr.com/post/48505329521/this-fucking-person-is-golden-all-of-the-awards
what are the legal ramifications of her actions if she was spiking her food with any substance, mild toxin or drug, just to get back the coworker?
If you poison the food it is assault. Intentionally causing harm to someone is illegal except in cases of self defense. Them stealing your lunch doesn’t give you the right to harm them. The note in the link is interesting. If you are in a jurisdiction where MJ is legal then IDK what may happen. But I do know it is a bad idea to post a note in public view at a workplace admitting to bringing a controlled substance into the workplace. Making the food taste gross would be legal. Though the best solution for me would be to stop putting my food where it gets stolen, fool me once. . .
What about putting a sleeping pill into the food?
So would the thief be able to sue if he had lost his job after the drug test?
That would fall under poisoning. What if the pill reacts with medication the person is on or they just fall asleep while driving home?
Could you spike it with laxatives, then claim you’ve been constipated and planned on eating the laxatives yourself?
Someone at the place I worked had a problem with her lunch being stolen. Finally she had had enough, and baked some “special” brownies, and really laced them with laxatives (I think she used Epsom salts). Sure enough, the thief stole them, very predictable, and also very amusing, results ensued. 
I would think “the food was for my own consumption, it’s his own fault for stealing food” would suffice as a valid defense. When you steal someone else’s lunch at work, you’re asking for it!
Sure he could sue, whether or not he could make a good case is questionable. The eater could just claim he ate the brownies by mistake (thought they were his, thought they were a gift, thought they were for the whole office, etc.), and that by bringing a controlled substance into the workplace and storing it in a place where it is likely to be consumed committed criminal negligence at least. If the note is allowed as evidence then the poisoner is admitting to the crime.
It’s not a valid defense. Intentional creating a likelihood of harm will make you liable.
Isn’t there an innocuous substance that makes your urine turn a lovely shade of turquoise blue? Can’t for the life of me remember what it is, if it even exists. (And yes, I have read Welcome to the Monkey House—I don’t think I’m confusing it with that.) It would seem to me that spiking said brownies with that compound would result in much ensuing hilarity…
Although deliberately scaring the living bejesus out of somebody is itself probably criminally sanctioned.
The easist solution would be to store your food elsewhere.
You could keep your food in a locked container and explain to others why you had to do so.
You could coat the food container with “Thief Detection Powder Ultraviolet” powder and use an ultraviolet light to detect the thief.
You could “spike” the food with an extremely hot/spicy “food grade” (it must be edible) condiment.
You could set up a video recorder to catch the thief but would probably need permission from “somebody” to do so.
Deliberately adding a non-food substance could be considered an assault and an attempt to poison someone. You could be criminally charged for doing so.
Vegetable dye, like the stuff you dye Easter eggs with. Dump enough in there and urine will be a nice, bright color. I recommend red in a dish like spaghetti sauce, which will send the person screaming for the ER. And it’s harmless.
Just for the record, writing a note doesn’t mean that the food was actually altered. It could be a ploy based on the fact that there was a drug test the previous day. The note writer wanted the food stealer to worry about being fired and maybe decide that they shouldn’t steal food in the future? OR that the food stealer would complain to management that they had been doped “by accident” which would result in self-identifying of the thief.
Good point
Just IMO but I’d bet most of these add X ideas (even the dye) are very bad ideas from a legal exposure sense. At best your sandwich with Ghost Pepper relish on it or that Live Maggot Cheese is about as far as you could go. And even with that you should probably at least be able to prove you can eat something like that yourself.
Don’t get me wrong. I think you SHOULD be able to do most these suggestions. Its just that I don’t think given current legal thinking and laws doing so would be anything other than a very bad idea that could seriously mess up your life.
And then you’ll get a bill from the insurance company for the cost of that E.R. visit.
Self-help is not a good idea. Don’t do it.
Suppose you made a sardine sandwich…and “left it out” for a few days? 
You’ll need to remember to mention, if questioned, that you really like vegetable dye on your food and didn’t intend it for anyone else. You have absolutely no idea how the thief could have come in contact with your favorite condiment - vegetable dye. hehehe.
It’s Methylene blue. It discolors urine and feces and has been mixed with psychiatric medications to monitor patient compliance with medication regimes. It’s the same stuff as in Welcome to the Monkey House. It has the advantage of having legitimate medical uses.
It is not completely innocuous, but is safe enough that it has been used as a placebo.
Two sorta-related stories. We were sitting in the breakroom when another employee walked in and said, “Damn it. I forgot I brought my lunch and just went out for a chicken dinner. Does anyone want my lunch?” He wasn’t normally such a generous person but someone said yes and grabbed the lunch bag from the refrigerator. It wasn’t five minutes later that the boss walked in, opened the refrigerator door and said, “Hey! Who took my lunch?” We were laughing so hard at the look on the face of the guy with his bosses sandwich in his mouth, I had tears in my eyes.
The 2nd story is something I read. A large family (lots-o-kids not overweight) were always hiding the last slices of pies and cakes from each other. Sometimes it worked, somtimes it didn’t. One daughter decide to toothpick a note to the last slice of pie that said - “I spit on this”. When she decided to eat her pie, she found a 2nd note, in her fathers handwriting, toothpicked to it that said - “So did I”. You can just feel the love. I don’t remember if anyone actually ate the pie.