I'm writing a Western, and in it, someone kills an entire table with a poison. I'm thinking arsenic, but I'm not sure if that was a very available poison back then. I'm not even sure how they were able to procure arsenic in the middle ages, when it was very much used (The King of Poisons, apparently). I know it can be found in minerals, but how it is isolated from those minerals, I don't know. Perhaps they didn't get their arsenic from the minerals?
Arsenic was used to treat syphilis. Now, penicillin came to replace it after the Wild West era. However, did doctors in the West have on them arsenic? And if so, would they have enough quantities of it to kill an entire table of people (30 people, big table). Lethal dose of arsenic is 2 to 20 mg/kg.
I think we can assume that there were be ample amount of arsenic available to kill people. It was a common by-product of milling gold and silver ore. There was a famous instance of this type of poisoning in (or rather from) the West; it's a little late for the classic "Wild West" but not that far off, and from it you can see the possibilities:
Missouri born, Cordelia Botkin (1854-1910) became nationally famous for sending a box of poisoned chocolates from California to Delaware, to the wife of her former lover, John Preston Dunning. Dunning was a reporter for the Associated Press who had a three-year affair with Botkin. Because of drinking, gambling, theft, and general dissipation, he lost his position. Dunning was rehired in early 1898 to report on the Spanish-American War. He subsequently reconciled with his wife, Mary Elizabeth Penington Dunning, before traveling to Cuba. Botkin was distraught over the loss of her lover and began sending threatening notes to Dunning's wife in Delaware. Finally, she sent her a box of arsenic-laden chocolates, which killed Mary Dunning and her sister, while sickening several others. Botkin was tried in San Francisco and convicted in December 1898. She was sentenced to life imprisonment in early 1899, but she appealed, was tried again and then was sentenced again to life imprisonment. Botkin died in San Quintin State Prison in 1910. John Dunning, who became unemployed with the end of the war, predeceased her as did her former husband and only son.
Sources: The New York Times (February 5, 1899); Oakland Tribune (Oakland, California; May 1, 1905); Washington Post (March 9, 1910).
u/itsallfolklore notes that arsenic would be a by-product of mining. After 1890, the MacArthur-Forrest Process would replace mercury amalgamation as a way to extract gold and silver. That required potassium cyanide, and while it was much better than mercury, it was quite dangerous- just a scratch on a hand at the wrong moment could be fatal. Whether the bitter almond smell of cyanide could be masked by rotgut whiskey depends on how far your poetic license extends.
There's also Paris or Emerald Green, a very bright green arsenical pigment beloved of Impressionist painters and for a time also used in other things, like green wallpaper. As the binder of the pigment in the wallpaper broke down, the pigment would get loose and begin to hurt people. That took time- much longer than even the longest poker game.
James C Whorton: The Arsenic Century: How Victorian Britain was Poisoned at Home, Work, and Play