This chapter begins the discussion of medicines as poisons and vice versa. It gives an overview of how the highly significant developments in the sciences in the nineteenth century, especially in chemistry, physiology and pharmacology, ‘opened new avenues for mass poisoning’ in many fields, including in hospitals. It focuses in particular on the important role of arsenic – the “king of poisons”, as John Parascandola has called it – in the deliberate poisoning of people. It investigates the story of a famous arsenic poisoner active in late-nineteenth-century Leiden, the Jekyll-and-Hyde character, ‘Good Mie’ and her alternate personality as ‘Murderous Mie’. Over the course of the nineteenth century, what had been seen as habituation to a strong drug gradually came to be seen as addiction and abuse. The pharmaceutical developments of the period have come to produce many more medicines that were quickly seen to be poisons.