A 35 year-old-man with known polysubstance abuse presents

with agitation and paranoid ideations. In the ED last night, it was noted that

he appears to be responding to internal stimuli - auditory and visual hallucinations. He is

reviewed this morning after a 14-hour admission to the 24 hour observation ED

ward. He has complete resolution of his symptoms and is apologetic for his

behaviour last night. He has no recollection of the events.

Which of the following is the most likely cause of his

behaviour?