Opioid agonists – how do they work?

Opioids are a category of painkillers primarily known for treating pain. These drugs will be divided into several categories: by chemical class, synthetic process, and motion on opioid receptors within the central and peripheral nervous system. There are three opioid receptors: delta (DOP), kappa (KOP), and mu (MOP), which, when activated, produce analgesic effects. Opioid agents act on these receptors as agonists, antagonists, or partial agonists. In this blog, we are going to concentrate on opioid agonists.

Opioid agnostics

Opioid agonists are medications used to treat severe pain that can’t be relieved by other medications or non-drug therapies. They will be divided into three chemical classes:

1. Phenanthrene opioid agonists: morphine, hydromorphone, codeine, oxycodone, oxymorphone and hydrocodone
2. Phenylpiperidine opioid agonists: meperidine, fentanyl, sufentanil, alfentanil and remifentanil
3. Diphenylhepatane opioid agonists: methadone and propoxyphene

How exactly do opioid agonists work to treat pain? Let’s delve into how they work.

Action mechanism

Opioid agonists inhibit pain transmission by mimicking the body’s natural pain control mechanisms. In the dorsal horn of the spinal cord, peripheral pain neurons meet neurons of the central nervous system (CNS). At the synapse, pain neurons release substance P (a pain neurotransmitter). This agent helps transmit pain impulses to CNS neurons, which carry these impulses to the brain. Theoretically, spinal interneurons reply to stimulation of descending CNS neurons by releasing endogenous opiates. These opiates bind to peripheral pain neurons, inhibiting the discharge of substance P and slowing down the transmission of pain impulses. Synthetic opiates complement this pain-blocking effect by binding to free opioid receptors, inhibiting the discharge of substance P. Opiates also alter pain awareness, but how this mechanism works stays unknown.

Side effects and monitoring

All opioids act, not less than partially, on the MOP receptor, and although MOP receptor agonism causes a lot of the analgesic effects of opioids, additionally it is answerable for many common unwanted effects (Pathan and Williams, 2012). Opioids may decrease the extent of consciousness, decrease the respiratory rate, suppress respiratory reflexes, and reduce systemic vascular resistance and blood pressure. Severe, life-threatening, or fatal respiratory depression may occur with opioid agonists. Additional unwanted effects include constipation, nausea, vomiting, urinary retention, itching, muscle stiffness, miosis, and dysphoria (Pathan and Williams, 2012).
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Using opioid medications puts people prone to addiction, abuse and misuse, which can lead to overdose and death. Health care providers should assess each patient’s risk before prescribing opioids and recurrently monitor all patients for the event of addictive behaviors (Facts & Comparisons, 2022).
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Full details will be present in the leaflet that comes with the drugs or within the Nursing2022 Medicines Manual + medicines updates.
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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. and Kaye, A. D. ( 2021). Full opioid agonists and tramadol: pharmacological and clinical considerations. ,(4),e119156. https://doi.org/10.5812/aapm.119156

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Facts and Comparisons (2022, February 28). Opioid painkillers. . https://fco.factsandcomparisons.com/lco/action/doc/retrieve/docid/fc_dfc/5545901?cesid=1zSW6hAD7hK&

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Pathan, H., and Williams, J. (2012). Basic opioid pharmacology: an update. (1), 11–16. https://doi.org/10.1177/2049463712438493

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