Opioid Pharmacology

Correct Answer

B — Partial mu opioid receptor agonist with high receptor affinity, available in sublingual and extended-release injectable formulations

Rationale

Buprenorphine is a partial agonist at the mu opioid receptor — it binds and activates the receptor but with submaximal intrinsic efficacy compared to full agonists such as morphine or fentanyl. It also has unusually high receptor affinity, meaning it binds mu receptors tightly and displaces full agonists. Formulations include sublingual tablets and film, a combination sublingual product with naloxone, and a monthly extended-release subcutaneous injection. A pure antagonist at mu, kappa, and delta receptors describes naloxone or naltrexone. A full mu agonist with N-methyl-D-aspartate antagonism and prolonged half-life describes methadone. A kappa agonist and mu antagonist available as a nasal spray describes butorphanol.

Correct Answer

D — Kappa opioid receptor agonist and mu opioid receptor antagonist classified as a mixed agonist-antagonist opioid

Rationale

Nalbuphine is a mixed agonist-antagonist opioid: it activates kappa receptors, producing spinal analgesia and sedation, while blocking mu receptors. Its mu antagonism is what distinguishes it from buprenorphine — whereas buprenorphine is a partial mu agonist, nalbuphine has no mu agonist activity and acts as a mu antagonist. This dual profile makes nalbuphine useful for treating opioid-induced pruritus by blocking the mu receptor pathway responsible for itch at the spinal cord level. A partial mu agonist with high affinity and a ceiling effect on respiratory depression describes buprenorphine. A pure antagonist with long duration describes naltrexone. A kappa agonist and partial mu agonist available as a nasal spray describes butorphanol.

Correct Answer

A — Kappa opioid receptor agonist and partial mu opioid receptor agonist, available as a nasal spray formulation

Rationale

Butorphanol is a mixed agonist-antagonist opioid that activates kappa receptors and has partial agonist activity at mu receptors. It is distinguished from nalbuphine by its partial mu agonism — nalbuphine has mu antagonist activity, while butorphanol has weak partial mu agonist activity. Butorphanol is available as an intranasal spray, making it one of the few opioids with a nasal delivery route for outpatient use. A kappa agonist combined with naloxone in an oral tablet to deter injection describes pentazocine. A pure mu antagonist for long-term maintenance describes naltrexone. A partial mu agonist with high affinity in sublingual and injectable formulations describes buprenorphine.

Correct Answer

C — Kappa opioid receptor agonist and weak mu opioid receptor antagonist, combined with naloxone in an oral tablet to deter injection misuse

Rationale

Pentazocine is a mixed agonist-antagonist opioid that activates kappa receptors and weakly antagonizes mu receptors. Because pentazocine tablets were historically crushed and dissolved for intravenous injection to exploit their opioid effects, the oral formulation was reformulated to include naloxone. When taken orally as intended, the naloxone is poorly absorbed and does not interfere with pentazocine's analgesic effect. If injected, the naloxone is bioavailable and precipitates withdrawal in opioid-dependent users, deterring misuse. A partial mu agonist with high affinity and a ceiling effect describes buprenorphine. A kappa agonist and partial mu agonist as a nasal spray describes butorphanol. A peripherally restricted mu antagonist for gastrointestinal motility describes methylnaltrexone or naloxegol.

Correct Answer

A — Pure opioid antagonist with longer duration of action than naloxone, approved for maintenance treatment of opioid use disorder and alcohol use disorder

Rationale

Naltrexone is a pure opioid antagonist — like naloxone, it has no intrinsic agonist activity at any opioid receptor — but it has a substantially longer duration of action, enabling once-daily oral dosing or monthly extended-release injectable administration. It is approved for long-term maintenance treatment of both opioid use disorder and alcohol use disorder, where it blocks the rewarding effects of any opioid taken during treatment. A partial mu agonist and kappa antagonist with high affinity describes buprenorphine. A peripherally restricted mu antagonist describes methylnaltrexone or naloxegol. A kappa agonist and mu antagonist for pruritus describes nalbuphine.

Correct Answer

D — Peripherally restricted mu opioid receptor antagonist derived from naloxone by polyethylene glycol modification, used to treat opioid-induced constipation

Rationale

Naloxegol is naloxone chemically modified by attachment of a polyethylene glycol chain, which increases its molecular size and reduces its ability to cross the blood-brain barrier. This peripheral restriction allows naloxegol to block mu receptors in the gastrointestinal tract — restoring bowel motility suppressed by opioids — without entering the central nervous system in clinically meaningful amounts. Because central mu receptors remain unblocked, analgesia is preserved and withdrawal is not precipitated. A pure antagonist with short duration for acute overdose reversal describes naloxone. A kappa agonist combined with naloxone in an oral tablet describes pentazocine. A partial mu agonist with high affinity and a ceiling effect describes buprenorphine.

Correct Answer

C — As a partial agonist, buprenorphine has submaximal intrinsic efficacy at mu receptors — above a threshold dose, additional drug produces no further increase in respiratory depression

Rationale

Intrinsic efficacy is the maximum receptor activation a drug can produce regardless of dose or receptor occupancy. A full agonist such as morphine or fentanyl has maximal intrinsic efficacy and produces proportionally greater respiratory depression as dose increases, with no upper limit until apnea occurs. Buprenorphine, as a partial agonist, has submaximal intrinsic efficacy — it activates mu receptors but cannot produce the same maximum receptor response as a full agonist even at doses that saturate all available receptors. This creates a ceiling: beyond a certain dose, respiratory depression plateaus at a level below that required for fatal apnea in most patients. This is the pharmacological basis for buprenorphine's improved safety margin in overdose. Buprenorphine's high affinity does not prevent it from reaching brainstem targets. It has kappa antagonist activity, not kappa agonist activity. Brainstem tissue does not selectively metabolize buprenorphine.

Correct Answer

A — Buprenorphine has higher mu receptor affinity than heroin's active metabolite morphine — it displaces morphine from receptors and, as a partial agonist, produces less receptor activation, triggering precipitated withdrawal

Rationale

Buprenorphine has exceptionally high affinity for the mu opioid receptor — higher than morphine, the active metabolite of heroin. When buprenorphine is administered while a full agonist still occupies receptors, buprenorphine competitively displaces the full agonist. Because buprenorphine is a partial agonist with submaximal intrinsic efficacy, the net effect of this displacement is a sharp reduction in mu receptor activation — from the high level maintained by a full agonist to the lower level produced by buprenorphine. This abrupt fall in opioid receptor tone precipitates acute withdrawal in a physically dependent patient. This is why buprenorphine induction requires waiting until the patient is in spontaneous mild to moderate withdrawal — at that point, receptor occupancy by the full agonist is low enough that buprenorphine's partial agonism represents an improvement over the current state. Buprenorphine has kappa antagonist activity, not kappa agonist activity. It does not inhibit norepinephrine reuptake. Hepatic competition is not the mechanism.

Correct Answer

B — Naloxone is poorly absorbed across sublingual mucosa and undergoes extensive first-pass hepatic metabolism when swallowed, resulting in negligible systemic bioavailability by the oral route

Rationale

Naloxone has very poor bioavailability when taken sublingually or swallowed. It is poorly absorbed across the sublingual and gastrointestinal mucosa, and any that is absorbed undergoes extensive first-pass hepatic metabolism before reaching systemic circulation. As a result, the naloxone in a buprenorphine/naloxone sublingual film produces negligible plasma concentrations and has no clinical effect on mu receptors when taken as prescribed. This is the pharmacokinetic basis for the abuse-deterrent design: if the combination product is dissolved and injected, naloxone bypasses first-pass metabolism entirely and reaches the systemic circulation at full concentration, where it can displace full agonists and precipitate withdrawal in opioid-dependent users. Buprenorphine's high affinity does not prevent naloxone from binding to receptors — naloxone is itself a high-affinity antagonist. Naloxone binds reversibly, not irreversibly. The naloxone dose is pharmacologically active by the intravenous route.

Correct Answer

D — Nalbuphine's mu receptor antagonist activity displaces methadone from mu receptors in a patient who is physically dependent, precipitating acute withdrawal

Rationale

Nalbuphine is a mu opioid receptor antagonist. When administered to a patient who is physically dependent on a full mu agonist such as methadone, nalbuphine competitively displaces methadone from mu receptors and blocks further mu receptor activation. In a physically dependent patient, the abrupt loss of mu receptor tone precipitates acute withdrawal — the same mechanism as giving naloxone to a dependent patient, but mediated by nalbuphine's mu antagonism rather than a pure antagonist. This reaction is a predictable consequence of giving any mu antagonist or mixed agonist-antagonist to a patient dependent on full mu agonists, and it represents a serious medication error in this clinical context. Mixed agonist-antagonist drugs are contraindicated in patients maintained on full mu agonists for this reason. The other options describe mechanisms that do not apply to this drug interaction.

Correct Answer

C — Nalbuphine partially blocks spinal cord mu receptors responsible for the centrally mediated itch signal while its kappa agonist activity contributes to analgesia, partially offsetting any analgesic reduction

Rationale

Opioid-induced pruritus from neuraxial opioid administration is centrally mediated through mu receptor activation in the spinal cord dorsal horn rather than through peripheral histamine release. Nalbuphine's mu antagonist activity partially competes with epidural morphine at these spinal mu receptors, reducing the itch signal. At the same time, nalbuphine's kappa agonist activity provides some analgesic contribution, which helps offset the partial reduction in mu-mediated analgesia from its mu antagonism. This balanced pharmacological profile allows nalbuphine to reduce pruritus without fully reversing postoperative analgesia at low doses. Nalbuphine does not block histamine receptors. While kappa agonism can contribute to anti-itch effects, the primary mechanism in this context is mu antagonism at the spinal cord. Nalbuphine does not redistribute morphine between spinal neuron populations.

Correct Answer

B — The oral patient receives analgesia because naloxone is poorly bioavailable orally; the person who injects the solution receives bioavailable naloxone that precipitates withdrawal if opioid-dependent

Rationale

The abuse-deterrent design of pentazocine/naloxone exploits the route-dependent bioavailability of naloxone. When taken orally, naloxone undergoes poor gastrointestinal absorption and extensive first-pass hepatic metabolism, reaching negligible plasma concentrations — it has no meaningful effect on mu receptors, and pentazocine provides its intended analgesic effect via kappa receptor agonism. When the tablet is dissolved and injected, naloxone bypasses first-pass metabolism entirely, reaching therapeutic plasma concentrations. In an opioid-dependent user, intravenous naloxone displaces opioids from mu receptors and precipitates acute withdrawal, making injection of the combination product aversive and deterring that route of misuse. Naloxone does not act synergistically with pentazocine. Crushing and dissolving does not destroy naloxone. Pentazocine's receptor selectivity does not change with route of administration.

Correct Answer

A — If residual physical dependence is present when naltrexone is initiated, its mu receptor blockade abruptly removes remaining opioid tone and precipitates severe acute withdrawal

Rationale

Naltrexone is a pure opioid antagonist that blocks mu receptors for 24 to 72 hours after a single dose. If a patient retains any degree of physical opioid dependence when naltrexone is initiated — because detoxification was incomplete — naltrexone's mu receptor blockade abruptly removes whatever residual opioid receptor activation was maintaining the neuroadapted state. This precipitates acute withdrawal, which can be severe and prolonged because naltrexone's long duration of action prevents re-opioidization until it is eliminated. The naloxone challenge test helps detect residual dependence before committing the patient to naltrexone's long-lasting blockade. Naltrexone can cause hepatotoxicity at very high doses but not in the context described here, and this is not why detoxification is required. Residual opioids do not inhibit the cytochrome P450 enzymes responsible for naltrexone metabolism in a clinically relevant way. Plasma protein binding competition is not the mechanism.

Correct Answer

D — Buprenorphine is a partial agonist with submaximal intrinsic efficacy — even at full receptor occupancy it cannot produce the same maximum response as a full agonist

Rationale

The plateau in buprenorphine's dose-response curve for respiratory depression reflects the concept of intrinsic efficacy. A full agonist such as morphine has maximal intrinsic efficacy — when it occupies a receptor, it produces the maximum possible conformational change and downstream signaling. A partial agonist such as buprenorphine has submaximal intrinsic efficacy — even when it occupies 100 percent of available mu receptors, it produces a smaller conformational change and a weaker signaling response than a full agonist at full occupancy. This means that no matter how much buprenorphine is administered, the maximum respiratory depression it can produce is limited by its intrinsic efficacy, creating the observed ceiling. Receptor saturation from high affinity does not create a ceiling effect — a high-affinity full agonist has no ceiling. Redistribution into adipose tissue does not explain the pharmacodynamic plateau. Buprenorphine has kappa antagonist activity, not kappa agonist activity at clinically relevant doses.

Correct Answer

B — Nalbuphine's mu receptor antagonist activity displaced heroin's active metabolites from mu receptors in a physically dependent patient, precipitating acute withdrawal

Rationale

Nalbuphine is a mu opioid receptor antagonist. When given to a patient who is physically dependent on heroin — even one who appears clinically well six hours after his last use — nalbuphine competes with and displaces morphine (the active metabolite of heroin) from mu receptors. Because the patient is neuroadapted to continuous mu receptor activation, the abrupt reduction in mu receptor tone caused by nalbuphine's antagonism precipitates acute withdrawal. This reaction illustrates why mixed agonist-antagonist drugs are contraindicated in patients dependent on full mu agonists and why a thorough history of opioid use is essential before administering any opioid to an emergency department patient. Kappa-mediated dysphoria is a feature of nalbuphine therapy but does not produce the autonomic and somatic features of opioid withdrawal. Nalbuphine does not inhibit cytochrome P450 2D6. Nalbuphine does not activate N-methyl-D-aspartate receptors.

Correct Answer

A — Buprenorphine's high mu receptor affinity displaced fentanyl still occupying receptors; as a partial agonist, buprenorphine then produced less receptor activation than fentanyl had been providing, precipitating withdrawal

Rationale

This case illustrates the mechanism of buprenorphine-precipitated withdrawal and why correct induction timing is essential. Because the patient used fentanyl recently and has no withdrawal symptoms, fentanyl and its metabolites are still occupying mu receptors and maintaining the neuroadapted opioid state. Buprenorphine has higher mu receptor affinity than fentanyl and competitively displaces it. However, because buprenorphine is a partial agonist, the degree of mu receptor activation it provides is less than what fentanyl was providing. This drop in receptor activation precipitates acute withdrawal in a physically dependent patient. The correct induction protocol — waiting until mild to moderate spontaneous withdrawal symptoms appear — ensures that the full agonist has dissociated from enough receptors that buprenorphine's partial agonism represents an improvement over the current low-opioid state. The naloxone in the sublingual formulation is poorly absorbed sublingually and reaches negligible systemic concentrations. Buprenorphine does not inhibit fentanyl metabolism. Buprenorphine has kappa antagonist activity, not kappa agonist activity.

Correct Answer

C — Naltrexone blocks mu receptors, making standard opioid doses ineffective — higher opioid doses, regional anesthesia, or non-opioid analgesics may be needed, with careful monitoring for respiratory depression when naltrexone eventually clears

Rationale

Extended-release naltrexone provides mu receptor blockade for approximately one month after injection. With the injection given 18 days ago, substantial receptor blockade is likely still present. Standard intraoperative opioid doses may be inadequate for analgesia because naltrexone occupies mu receptors competitively, and very high doses of opioids would be needed to overcome this blockade — with the risk that as naltrexone gradually clears over the following days, the patient could experience delayed respiratory depression from residual opioid. Regional anesthesia techniques and non-opioid analgesics are preferred where possible. This clinical scenario is one of the important practical considerations in naltrexone maintenance and highlights the need for patients to inform all treating clinicians of their naltrexone use. Naltrexone is not metabolized by general anesthesia. It does not cause serotonin syndrome. Naltrexone is a pure antagonist with no kappa agonist activity.

Correct Answer

D — Intravenous naloxone from the dissolved film reaches systemic circulation with full bioavailability, displacing buprenorphine and precipitating withdrawal in a physically dependent patient

Rationale

This case demonstrates the abuse-deterrent mechanism working as designed. When the buprenorphine/naloxone film is dissolved and injected, naloxone — which is poorly bioavailable sublingually and orally — enters the systemic circulation with nearly complete bioavailability. Naloxone is a high-affinity pure mu antagonist. At systemic concentrations, it competes with and displaces buprenorphine from mu receptors. Although buprenorphine also has high mu receptor affinity, naloxone can achieve sufficient receptor occupancy to reduce the net mu receptor activation below the level maintained by buprenorphine alone, precipitating acute withdrawal in a patient dependent on buprenorphine maintenance. The symptoms — agitation, diaphoresis, cramping, vomiting — are classic opioid withdrawal driven by the abrupt reduction in mu receptor tone. Intravenous buprenorphine produces opioid effects, not toxicity, when not combined with intravenous naloxone. Buprenorphine's pharmacological profile does not change with route of administration.