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The two things that every would-be killer should know about cyanide are these: It works. Incredibly well. And it works incredibly fast. These are not especially new insights, of course. A 1923 forensics book calls cyanide poisons “one of the most rapidly fatal poisons known.” The victim’s good health, the text continues, offers no protection at all: “… in many cases of poisoning the victims have been found dead within a very short time after they had been seen perfectly well.”