A 72-year-old man with Parkinson disease is brought by his wife because he has begun speaking to people who are not present and has become suspicious of her. He remains oriented, has no fever, and his motor symptoms have not worsened. His medicines are levodopa–carbidopa, pramipexole and procyclidine; the procyclidine was added 10 days ago for troublesome tremor. Which one of the following is the most appropriate next step?