Which of the following opioid analgesics is most likely to cause central nervous system (CNS) disturbance?

Meperidine has a half-life of 3 hours, but it has an active metabolite, normeperidine, which has a half-life of 15-20 hours. Therefore, accumulation of normeperidine with toxicity is common. The drug should not be used for prolonged periods (over 48 hours), and probably should not be used at all in those susceptible to delirium (e.g., the elderly).