Opioids are a class of analgesic agents known primarily to treat pain. These drugs can be categorized in several ways: by chemical class, by synthetic process, and by their action on opioid receptors in the central and peripheral nervous systems. There are three opioid receptors: delta (DOP), kappa (KOP), and mu (MOP), which when activated, cause analgesia. Opioid agents act at these receptors either as agonists, antagonists, or partial agonists. In this blog we will focus on the opioid agonists.
Opioid agonists are medications used to treat severe pain that can’t be managed by other drugs or non-pharmacologic therapies. They can be grouped into three chemical classes:
- Phenanthrene opioid agonists: morphine, hydromorphone, codeine, oxycodone, oxymorphone, and hydrocodone
- Phenylpiperidine opioid agonists: meperidine, fentanyl, sufentanyl, alfentanil, and remifentanyl
- Diphenylhepatane opioid agonists: methadone and propoxyphene
How exactly to opioid agonists work to treat pain? Let’s dive deeper into their mechanism of action.
Mechanism of Action
Opioid agonists inhibit pain transmission by mimicking the body’s natural pain control mechanisms. Within the dorsal horn of the spinal cord, the peripheral pain neurons meet the central nervous system (CNS) neurons. At the synapse, the pain neurons release substance P (a pain neurotransmitter). This agent helps transfer pain impulses to the CNS neurons that carry those impulses to the brain. In theory, the spinal interneurons respond to stimulation from the descending neurons of the CNS by releasing endogenous opiates. These opiates bind to the peripheral pain neurons to inhibit release of substance P and to slow the transmission of pain impulses. Synthetic opiates supplement this pain blocking effect by binding with free opiate receptors to inhibit the release of substance P. Opiates also alter consciousness of pain, but how this mechanism works remains unknown.
Side Effects & Monitoring
All opioids act, at least in part, on the MOP receptor, and while MOP receptor agonism produces the majority of analgesic effects of opioids, it also accounts for many of the common side-effects (Pathan & Williams, 2012). Opioids may reduce the level of consciousness, depress the respiratory rate, dampen the airway reflexes, and decrease systemic vascular resistance and blood pressure. Serious, life-threatening, or fatal respiratory depression may occur with the use of opioid agonists. Additional side effects include constipation, nausea, vomiting, urinary retention, pruritus, muscular rigidity, miosis, and dysphoria (Pathan & Williams, 2012).
Use of opioid drugs exposes individuals to risks of addiction, abuse, and misuse, which can result in overdose and death. Health care providers should asses each patient’s risk prior to prescribing opioids, and monitor all patients regularly for the development of addictive behaviors (Facts and Comparisons, 2022).
For complete information, please consult the drug’s specific package insert or the Nursing2022 Drug Handbook® + Drug Updates
Edinoff, A. N., Kaplan, L. A., Khan, S., Petersen, M., Sauce, E., Causey, C. D., Cornett, E. M., Imani, F., Moradi Moghadam, O., Kaye, A. M., & Kaye, A. D. (2021). Full Opioid Agonists and Tramadol: Pharmacological and Clinical Considerations. Anesthesiology and pain medicine, 11(4), e119156. https://doi.org/10.5812/aapm.119156
Facts and Comparisons (2022, February 28). Opioid Analgesics. Facts and Comparisons. https://fco.factsandcomparisons.com/lco/action/doc/retrieve/docid/fc_dfc/5545901?cesid=1zSW6hAD7hK&
Pathan, H., & Williams, J. (2012). Basic opioid pharmacology: an update. British journal of pain, 6(1), 11–16. https://doi.org/10.1177/2049463712438493
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