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Pharmacology · chapter 14 · Pain Response Drugs

Opioid Agonists and Antagonists

The 14 things this section of the textbook says that you are most likely to be asked about — quoted word for word, not retold. Then 6 practice questions with the reasoning.

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Key points

  1. Most opioids are Schedule II

    Most opioids fall under the strict categorization of Schedule II (CII), meaning they have an acceptable medical use but are associated with a high risk for misuse and addiction.

  2. Same receptors, dangerous effects

    These same receptors are also known to cause many of the notable adverse effects of opioids, including the CNS and respiratory depression that can be seen in overdose.

  3. Codeine in rapid metabolizers

    Clients producing many copies of CYP2D6 (rapid metabolizers) may convert too much codeine into morphine, causing enhanced analgesic effects and a greater risk for life-threatening respiratory depression.

  4. Fentanyl and morphine allergy

    Fentanyl is considered a synthetic opioid because it shares no chemical resemblance to naturally occurring opioids such as morphine, which means it can be a safe alternative for clients who have a documented morphine allergy.

  5. Why meperidine is rarely used for pain

    This is because it produces a toxic metabolite that can accumulate in older adults with poor renal function, leading to risk for delirium and seizures.

  6. Oxycodone combined with acetaminophen

    Oxycodone is often coformulated with acetaminophen, which nurses should consider when totaling a client’s total daily acetaminophen intake.

  7. Constipation with chronic opioids

    Nearly all clients receiving chronic opioid therapy will develop constipation, so nurses should assess for this to help determine which type of constipation treatment (laxatives, stool softeners, stimulants, fiber supplements) the client needs to ensure regular bowel movements.

  8. NARCS U mnemonic

    A handy mnemonic for adverse effects is NARCS U: nausea, acute toxicity/addiction, respiratory depression, constipation, sedation, and urinary retention.

  9. Morphine and histamine release

    Morphine is known to cause significant histamine release in susceptible clients and may manifest as flushing, pruritis, and hypotension.

  10. Recognising opioid overdose

    The individual may appear sedated or can be completely unresponsive, and their breathing may be slow, shallow, or absent. These signs constitute a medical emergency and necessitate the use of an opioid antagonist such as naloxone hydrochloride (discussed in the following section) along with the response of emergency medical services.

  11. Who should avoid opioids

    Clients with significant respiratory depression or gastrointestinal obstruction should also avoid opioids because these conditions can worsen when opioids are present in the body.

  12. Tolerance is not addiction

    This tolerance is a normal phenomenon that occurs in nearly all clients receiving chronic opioid agonists and should not be confused with addiction.

  13. Naloxone wears off before many opioids

    Importantly, naloxone has a duration of action of only approximately 20–30 minutes. This short duration means the individual may resedate because many opioids have a longer duration of action than naloxone does.

  14. Naloxone can trigger withdrawal

    The only major adverse effect of naloxone is that it can induce severe opioid withdrawal in individuals who chronically use opioids; they may become very agitated, develop diarrhea, and experience intense dysphoria.

Terms to know

opiate
The term opiate strictly refers to agents that come from the opium poppy (e.g., heroin, morphine), but is often used interchangeably with the term opioid.
Tolerance
Tolerance is a phenomenon that occurs in clients taking opioid agonists chronically.
opioid use disorder
Opioid addiction (also called opioid use disorder) is a condition characterized by compulsive use of opioids, increased opioid tolerance, and withdrawal symptoms if the client does not continue taking an opioid agonist.
Naloxone hydrochloride
Naloxone hydrochloride is an opioid receptor antagonist that is used as an antidote in people experiencing signs and symptoms of opioid agonist overdose, including severe CNS and respiratory depression.

Practice questions

Stuck on select-all-that-apply? How to take them one option at a time.

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Every question here, with the answer

  1. A client receiving IV morphine after surgery is difficult to arouse and has slow, shallow breathing. What is the nurse's priority action?

    1. Document the finding and recheck in one hour.
    2. Give the next morphine dose early to prevent pain.
    3. Give an opioid antagonist such as naloxone and call for emergency help.
    4. Offer the client oral fluids and fiber.

    Answer: Give an opioid antagonist such as naloxone and call for emergency help.

    Sedation with slow, shallow or absent breathing is a medical emergency that requires an opioid antagonist such as naloxone plus emergency response. Waiting, giving more opioid or offering fluids does not reverse respiratory depression.

  2. A client who received naloxone for an opioid overdose is now awake and breathing well and wants to go home. Why should the nurse encourage the client to stay for further care?

    1. Naloxone causes severe constipation.
    2. Naloxone lasts only about 20–30 minutes, so the client may become sedated again.
    3. Naloxone must be followed by a second opioid dose.
    4. Naloxone causes permanent sedation after an hour.

    Answer: Naloxone lasts only about 20–30 minutes, so the client may become sedated again.

    Naloxone acts for only about 20–30 minutes, and many opioids last longer, so the client may resedate and needs medical care. The other options describe effects naloxone does not have.

  3. A nurse is teaching a client who is going home with an opioid agonist prescription. Which instructions should the nurse include? Select all that apply.

    1. Share leftover tablets with family members who have pain.
    2. Do not crush or chew extended-release tablets.
    3. Take a double dose if you miss one.
    4. Keep the medication out of the reach of children and pets.
    5. Do not drink alcohol or take sedatives or muscle relaxants with this drug.

    Answer: Do not crush or chew extended-release tablets., Keep the medication out of the reach of children and pets., Do not drink alcohol or take sedatives or muscle relaxants with this drug.

    Clients must not crush or chew extended-release products, must not mix opioids with other CNS depressants because of the risk of respiratory failure, and must keep opioids away from children and pets. Sharing opioids and doubling doses are specifically warned against.

  4. A client with a documented morphine allergy has severe pain after trauma. Which opioid does the nurse expect may be a safe alternative?

    1. Fentanyl
    2. Codeine
    3. Heroin
    4. Morphine at a lower dose

    Answer: Fentanyl

    Fentanyl is synthetic and chemically unlike naturally occurring opioids such as morphine, so cross-sensitivity is low. Codeine and heroin are naturally occurring opioids like morphine, and a lower morphine dose does not avoid an allergy.

  5. Which findings should the nurse monitor for in a client on chronic opioid therapy? Select all that apply.

    1. Urinary retention
    2. Diarrhea as an expected daily effect
    3. Respiratory depression
    4. Constipation
    5. Sedation

    Answer: Urinary retention, Respiratory depression, Constipation, Sedation

    The NARCS U mnemonic lists nausea, acute toxicity or addiction, respiratory depression, constipation, sedation and urinary retention. Opioids cause constipation, not diarrhea; diarrhea is a sign of withdrawal.

  6. A client with cancer pain says the same morphine dose no longer works and fears becoming addicted. What is the nurse's best explanation?

    1. Needing a higher dose always means addiction.
    2. Tolerance only happens in people who misuse opioids.
    3. The client should stop morphine immediately.
    4. Tolerance is a normal effect of chronic opioid use and is not the same as addiction.

    Answer: Tolerance is a normal effect of chronic opioid use and is not the same as addiction.

    Tolerance occurs in nearly all clients on chronic opioids and should not be confused with addiction, which involves compulsive use. Stopping abruptly risks withdrawal, and tolerance is not limited to misuse.

Where every quote comes from

Section 14.3 Opioid Agonists and Antagonists of Pharmacology for Nurses by Tina Barbour-Taylor, Leah Mueller (Sabato), Donna Paris, Dorie Weaver, OpenStax, 2024. Read the whole section free at openstax.org.

The textbook is licensed under CC BY-NC-SA 4.0. This page quotes it word for word and adds the headings, the order and the practice questions; references to the book's figures and stray spaces left by its formatting are removed. The page is shared under the same licence. Nothing here is sold.