PHARMACOLOGY OF OPIOIDS

Classification of Opioid Compounds

Opioids can be classified as naturally occurring, semisynthetic, and synthetic ( Table 11-1 ). The naturally occurring opioids can be divided into two chemical classes, phenanthrenes (morphine and codeine), and benzylisoquinolines (papaverine). The semisynthetic opioids are morphine derivatives in which one of several changes has been made. Synthetic opioids are classified into four groups, the morphinan derivatives (levorphanol), the diphenyl or methadone derivatives (methadone, d-propoxyphene), the benzomorphans (phenazocine, pentazocine), and the phenylpiperidine derivatives (meperidine, fentanyl, alfentanil, sufentanil, and remifentanil). Structures of opioid compounds are shown in Figure 11-1 and Table 11-2 . ^[1] Opioids can be classified as agonists, partial agonists, mixed agonist-antagonists and antagonists on the basis of their interaction with the opioid receptors.