A 79-year-old man has fallen three times in two months, usually when walking to the toilet at night. He has slowed reactions and feels unsteady for several hours after waking, but has no focal neurological signs or postural blood pressure fall. His medicines are amlodipine, diazepam 5 mg at night, furosemide, metformin and sertraline; diazepam was continued after it was prescribed for bereavement-related insomnia 18 months ago. Which one of the following medicines is most likely to be contributing to his falls?