In the case of Kubla Khan, I believe the story is Coleridge was
called away on that errand by a
man from Porlock, wherever that is. That man from Porlock has gone down in the history of writing, and other creation, as whatever calls us away in the middle of it.
As for the myriad of drugs Coleridge used, the main one seems to have been laudanum, a tincture of opium--a solution in alcohol. Opium, as all opiates, I believe, have the effect of slowing down the bowel, and relieving various complaints.
Unlike today, there were no limitations on what could be prescibed to people. There were occasional tragedies.