A 69-year-old man presents with acute confusion, visual hallucinations, and fluctuating levels of consciousness developing over the past 2 days. He has Parkinson's disease treated with Levodopa/Carbidopa. He was recently started on co-codamol 30/500 for back pain. On examination, he is disorientated, has a tremor, and mild rigidity. His temperature is 37.2°C, and observations are otherwise stable. What is the most likely cause of his presentation?