A poison given to the accused to determine guilt or innocence. Like a witch trial, the idea behind the ordeal poison was that the guilty would die and the innocent would live when dosed.
I had always thought that ordeal poisons were another example of superstition and dubious science until I ran across a webpage which said explained how the Calabar Bean (Physostigma venosum) was used as an ordeal poison in (what is now) Nigeria:
The traditional use of this poison has some scientific basis since, if the infusion was
nervously sipped by the guilty, a considerable amount of physostigmine would be absorbed
though the gut. On the other hand, if an innocent person boldly drank a large volume, this
would result in rapid violent vomiting and little of the alkaloid would be absorbed.
(For the curious, physostigmine is an alkaloid which slows neurotransmitter break down, leading to over-exicitation of the muscles and brain. Currently physostigmine is being used to try to treat Alzheimer's Disease)
So maybe ordeal poisons did work as a psychological experiment, rather than a physiological one.
Me, I'll take my chances with a jury.