The Calabar bean is the plant whose seeds supplied the West African ordeal poison containing physostigmine. Its scientific name is Physostigma venenosum, a climbing legume native to tropical Africa, especially the region historically called Old Calabar in present-day Nigeria.
Physostigmine is a reversible inhibitor of acetylcholinesterase, the enzyme that normally breaks down the neurotransmitter acetylcholine. Excess acetylcholine can produce salivation, vomiting, seizures, loss of bladder and bowel control, respiratory failure, and death. The bean’s effects made it central to ordeal trials, in which survival could be interpreted as proof of innocence.
The plant is also called the ordeal bean, and physostigmine was once known as eserine, a name derived from éséré, a West African term for the bean. It later became medically useful, including in treatments involving glaucoma and certain toxic syndromes. The Strychnine tree, rosary pea, and castor bean are poisonous plants, but they do not provide this canonical combination of ordeal use and physostigmine.