Opium was an incredibly popular drug during the romantic period. Opium was used for a wide variety of ailments, but Samuel Taylor Coleridge started using it to elevate the pain he felt in his knees. He soon became addicted to opium and had a plethora of reasons, real or imagined, to keep taking it. Opium caused its takers to enter a dream-like state. Some argue that opium inspires the mind in such a way that an author could achieve their best writing. Others argue that opium would merely unleash the creativity already inside of the author. For Coleridge, he believed that "opium made his body harmonize with his soul". Either way, opium had a heavy influence on many authors during the romantic period. There is heavy debate that Kubla Khan was written by Coleridge under the influence of opium. He was prescribed some sort of drug for his illness, had a vivid dream for three hours, awoke to write the verses found above, but was then distracted and was unable to finish the poem or recollect the rest of the dream. Personally, I am in the camp that believes it was an opium-induced dream.
Sources:
https://en.wikipedia.org/wiki/Opium_and_Romanticism
https://www.bl.uk/romantics-and-victorians/articles/kubla-khan-and-cole…
https://www.bl.uk/romantics-and-victorians/articles/representations-of-…
https://en.wikipedia.org/wiki/Coleridge_and_opium
http://www.bbc.co.uk/devon/discovering/famous/coleridge.shtml