White Snakeroot

Though we have not seen it used in cocktails, this plant is poisonous. 

According to the book Wicked Plants [link], White Snakeroot contains the toxin tremetol. Cattle/animals eat plants, the meat and milk become toxic, and this can cause tremetol poisoning in humans. The poisoning is also called milk sickness, as humans often ingested the toxin by drinking the milk of cows that had eaten snakeroot.