When a recipe refers to laudanum, it really means opium. Laudanum is a tincture of opium prepared by dissolving extracts of the opium poppy. It was, and remains, a very strong narcotic and up to the 40 drops suggested in a recipe for a sleep aid, it might do a little more than give you some sweet dreams. Many of the recipes also just refer to opium, whether it be a solution or a tincture. Opium and its derivatives have been used in medicine since before 500 AD. As a potent form of pain relief, it was a widespread and commonly used narcotic. In comparison to many other chemicals used by medical practitioners like mercury and arsenic, its use as a sedative is perhaps not quite as toxic.
During the eighteenth century, as illustrated by these recipes, it was often used for sedation. Now highly illegal, it’s interesting how use of opium has developed from feeding it to babies to quieten their cries to being a restricted drug alongside all of its derivatives like heroin.