Sometimes you don’t need to add anything to make it work. In the late 60s I was a restaurant cook and we frequently had sodas or milkshakes nearby to stay cool in the kitchen heat. One guy who worked with us had the bad habit of grabbing someone else’s shake and sipping rather than buying his own. One evening we all started snickering, smiling and staring at him after he had finished half of someone’s shake. He demanded that we tell him what was going on and one guy took out a half-used chocolate ex-lax bar and we all started laughing and asked if the chocolate shake was good. We told him to not hesitate if he felt any twinge during the shift. Now we had put nothing into the shake but the power of suggestion kicked in and he spent most of the rest of the shift running to the restroom and cursing us.
In high school, we had students who paid a part of their tuition by working behind the lunch counter, under the supervision of the adult kitchen staff. One time, one of them dumped a bottle of chocolate-flavored Haley's M-O (for "Mineral Oil"?) into the chocolate soft-serve ice cream machine.
It didn't take the staff long to figure out what was causing the mad rush to the restroom, but by then the culprit had disposed of the evidence and denied all. I think that batch of ice cream was all gone anyway, replaced by another untainted supply. Much later, the guy 'fessed-up to one of his buddies and word got around. I thought it was funny as could be, but then I didn't eat any ice cream that day.