Laudanum is a solution of opium and alcohol with pain-relieving properties, first invented in a rough form by 16th century alchemist Paracelsus. Rediscovered in the late 17th century, by Shelley's time it was widely available and prescribed enthusiastically as a treatment for various physical and psychological ailments, including insomnia. Opium's highly addictive properties were not yet fully recognised; its medicinal formulation as laudanum allowing respectable members of society to dabble in its recreational use.
Some quietly nurtured lifelong habits, like Romantic poet Samuel Taylor Coleridge (See note to page 10), whose work is consequently sometimes regarded as having emerged from a drug-fuelled haze. The alluring myth of the artist using exotic substances as a swift pathway to inspiration has remained popular, although Frankenstein's "double dose" probably had darker associations in his creator's mind - her half-sister Fanny Imlay, following Mary's flight to Europe, had killed herself with a lethal overdose in October 1816.
Online edition of Thomas de Quincey's 'Confessions of an English Opium Eater', (1822)