Absinthe is a highly alcoholic spirit, usually 45-75% alcohol. It was considered an extremely addictive and dangerous psychoactive drug. Traditionally, it contained a chemical called thujone, which can be blamed for these properties; thujone is not present in modern absinthe.
Absinthe is famously linked to New Orleans because of a building there called the Old Absinthe House, which Oscar Wilde and Mark Twain are said to have visited.
Laudanum is a preparation containing opium and morphine. Although it was, and still is, used as a medicine, as with any drug it is harmful if consumed in the wrong quantity. It is a poison, and can kill.