At an unknown point in time, the toxin was either developed or discovered by the British Men of Letters who began utilizing it in the field to take down monsters. The toxin and its antidote were taught to all of the organization's operatives.
In 2018, a bounty hunter in Apocalypse World utilized a variation of the toxin on hunter Dean Winchester. Former British Men of Letters operative Arthur Ketch, who was traveling with Dean, recognized the toxin and was able to create the antidote to save Dean's life.
The toxin is known to cause the target to develop a high fever and weakness over time. The fever causes delirium in the victim as well. The site of injection turns black and black veins spread outwards around it. If an antidote isn't administered, the victim will suffer a particularly gruesome death.
The differences between the British version and the Apocalypse World one are unknown, but are extremely similar in nature to the point that Arthur Ketch identified the toxin as being similar to the British version on sight and the same antidote worked for both. The only known difference is that the bounty hunter that used the toxin coated his bullets in it to hobble his prey.
The antidote contains taro root, a trace of arsenic and basidiomycota washed in holy water. The ingredients are then ground into a paste that is applied to the wound, neutralizing the toxin though it takes awhile for the subject to regain their strength.