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Derivatives of the opium poppy have been used to relieve severe pain for hundreds (possibly thousands) of years. Morphine, the prototypic opioid agonist, does so with remarkable efficacy. This alkaloid (named after Morpheus, the Greek god of dreams) is extracted from crude opium, which is obtained from the opium poppy seed pod. Morphine remains the standard against which all drugs that have strong analgesic action are compared in terms of efficacy and potency. These drugs are collectively known as opioid analgesics and include not only the natural opium alkaloids and semisynthetic alkaloid derivatives from opium, but also synthetic surrogates (opioid-like analgesic drugs whose actions are blocked by the nonselective antagonist naloxone), and endogenous peptides that interact with several opioid receptor subtypes. Opioid analgesics are characterized by their ability to relieve moderate to severe pain. Many opioids also have useful antitussive or antidiarrheal effects.

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Analgesic drug therapy and many physical therapy interventions are aimed at the same outcome: pain relief. For this reason, analgesic drugs are among the most frequently prescribed medications for patients receiving rehabilitative therapy. It is essential for physical therapists to be familiar with the intended uses and common adverse effects of opioid analgesics to effectively monitor patient outcomes. The opioid medications are classified as controlled substances because of their potential for abuse; therefore, an understanding of the clinical signs of physical dependence and tolerance is also needed. The major subclasses of opioid agonist and antagonist drugs are outlined in Figure 20–1.

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Figure 20–1.
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Major subclasses of opioid agonists and antagonists.

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The other major group of analgesic drugs comprises aspirin and the other nonsteroidal anti-inflammatory drugs (NSAIDs). The NSAIDs have a significantly lower maximal efficacy than the opioids and have no addiction liability. Therefore, they are not considered narcotics and are available with an ordinary prescription or even over-the-counter. The NSAIDs are discussed in Chapter 34.

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Although they are not peptides, opioid alkaloids (e.g., morphine) produce analgesia through actions in regions of the brain that contain endogenous peptides which have opioid-like pharmacologic properties. The general term currently used for these substances is endogenous opioid peptides, which replaces the previous term endorphins. Three families of endogenous opioid peptides have been described: enkephalins, dynorphans, and endorphins. These peptides are synthesized in the soma of neurons and transported to the nerve endings where they accumulate in synaptic vesicles and are released from nerve terminals. Although chemically quite different from the opioid alkaloids, these endogenous peptides bind to opioid receptors and modulate transmission in the brain and spinal cord, and at the primary afferents. It remains unclear whether these peptides function as classic neurotransmitters or as modulator neuropeptides. Opioid peptides are also found in the adrenal medulla and the neural plexus (enteric nervous system) of the gut.

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The opioid analgesics and related ...

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