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INTRODUCTION

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 euphoric effects, μ-opioid agonists are important drugs of abuse and are responsible for severe personal and social problems.

Opioid peptides released from nerve endings modulate neurotransmission in the brain, spinal cord, and in primary afferents via their interaction with one or more of the above opioid receptors.

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CLASSIFICATION

The opioid analgesics and related drugs are derived from several chemical subgroups and may be classified in multiple ways.

A. Spectrum of Clinical Uses

Opioid drugs can be subdivided on the basis of their major therapeutic uses (eg, analgesics, antitussives, and antidiarrheal drugs).

B. Strength of Analgesia

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, (ie, have decreased efficacy) as compared to the prototype full agonist, morphine.

C. Ratio of Agonist to Antagonist Effects

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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High-Yield Terms to Learn
Opiate A drug derived from alkaloids of the opium poppy
Opioid The class of drugs that includes opiates, opioid peptides, and all synthetic and semisynthetic drugs that mimic the actions of the opiates
Opioid peptides Endogenous or synthetic peptides that act on opioid receptors
Opioid agonist A drug that activates some or all opioid receptor subtypes and does not block any
Partial agonist A drug that can activate an opioid receptor with a submaximal response at maximal dosage
Opioid antagonist A drug that blocks some or all opioid receptor subtypes
Mixed agonist-antagonist A drug that activates some opioid receptor subtypes and blocks other opioid receptor subtypes

PHARMACOKINETICS

A. Absorption and Distribution

Most drugs in this class are well absorbed when taken orally, but morphine, hydromorphone, and oxymorphone undergo extensive first-pass metabolism. In most ...

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