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The opioids include natural opiates and semisynthetic alkaloids derived from the opium poppy, pharmacologically similar synthetic surrogates, and endogenous peptides. On the basis of their interaction with several opioid receptors (μ[mu], δ [delta], and κ [kappa] receptors), individual drugs are classified as agonists, mixed agonist-antagonists, and antagonists at one or more of these receptors. Because of their psychic effects, opioids are important drugs of abuse and are responsible for severe personal and social problems.
Opioid peptides (opiopeptins) released from nerve endings modulate transmission in the brain and spinal cord and in primary afferents via their interaction with one or more of the above receptors. Most of the pharmacologic actions of opiates and synthetic opioid drugs are effected via their interactions with opioid peptide receptors.
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The opioid analgesics and related drugs are derived from several chemical subgroups and may be classified in several ways.
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A. Spectrum of Clinical Uses
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Opioid drugs can be subdivided on the basis of their major therapeutic uses (eg, analgesics, antitussives, and antidiarrheal drugs).
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B. Strength of Analgesia
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On the basis of their relative abilities to relieve pain, the analgesic opioids may be classified as strong, moderate, and weak agonists. This classification is independent of potency. Strong agonists vary markedly in potency; thus morphine is only one-hundredth as potent as fentanyl (0.1 mg fentanyl is as analgesic as 10 mg morphine), and is estimated to be less than one-thousandth as potent as carfentanil, a veterinary large animal opioid that has recently been detected as an adulterant in street heroin. Partial agonists are opioids that exert less analgesia, regardless of dosage, than morphine, the prototype of a strong analgesic, or full agonist.
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C. Ratio of Agonist to Antagonist Effects
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Opioid drugs may be classified as agonists (full or partial receptor activators), antagonists (receptor blockers), or mixed agonist-antagonists, which are capable of activating one opioid receptor subtype and blocking another subtype.
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A. Absorption and Distribution
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Most drugs in this class are well ...