And I don't think there was an antidote widely known at this point in the novel's timeline, 1811. If any of you could assist me in determining what Sonya might have administered as an antidote, I'd be grateful.
Arsenic was widely available as rat poison after the mass production of arsenic oxides as a byproduct of smelting in the late 1700's. Through Google Scholar, I've found one reference to recommended "treatments" in 1806: /3