Figure 11-1 Chemical structures of piperidine and phenylpiperidine analgesics. (From Gutstein HB, Akil H: Opioid analgesics. In Hardman JG, Limbird LE [eds]: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 10th ed. New York, McGraw-Hill, 2001, pp 569–619.)