Back in the ‘90s, there was a guy at work who was a real POS because he was a recalcitrant food thief. He would raid the fridge for whatever suited him, and he seemed to love stealing lunchmeat sandwiches.
Some other guys at the shop prepared a deluxe sandwich with methylene blue granules from the dye lab, and it worked a treat. The thief lost his shit and went to the ER whereupon he was told what had happened.