Opioid Pharmacology

Correct Answer

C — Full mu opioid receptor agonist with N-methyl-D-aspartate receptor antagonist activity, dispensed only through federally licensed opioid treatment programs, with QTc interval monitoring required

Rationale

Methadone is a full mu opioid receptor agonist belonging to the phenylheptylamine chemical family, with additional N-methyl-D-aspartate receptor antagonist properties that contribute to its efficacy in neuropathic pain and its complex tolerance profile. For opioid use disorder treatment, methadone can only be dispensed through federally licensed opioid treatment programs — it cannot be prescribed for this indication in a standard office-based setting. Monitoring of the QTc interval is required because methadone prolongs cardiac repolarization through hERG potassium channel blockade. A partial mu agonist dispensed through office-based prescribers describes buprenorphine. A pure antagonist with a monthly extended-release formulation describes extended-release naltrexone. A synthetic phenylpiperidine with transdermal formulation describes fentanyl.

Correct Answer

A — Partial mu opioid receptor agonist combined with a naloxone abuse deterrent, available for office-based prescribing without a federal waiver

Rationale

Buprenorphine/naloxone is a sublingual combination product in which buprenorphine acts as the active partial mu opioid receptor agonist and naloxone serves as an abuse deterrent — poorly absorbed sublingually but bioavailable if injected, precipitating withdrawal. A landmark regulatory change removed the requirement for a special federal Drug Addiction Treatment Act waiver, allowing any clinician with a standard Drug Enforcement Administration registration to prescribe buprenorphine for opioid use disorder in an office-based setting. This expanded access substantially. A full mu agonist dispensed only through licensed opioid treatment programs describes methadone. A pure antagonist requiring detoxification given as a monthly injection describes extended-release naltrexone. A peripherally restricted antagonist for constipation describes methylnaltrexone or naloxegol.

Correct Answer

D — Pure mu opioid receptor antagonist administered as a monthly intramuscular injection, requiring complete opioid detoxification before initiation

Rationale

Extended-release naltrexone is a pure opioid antagonist formulated as a monthly intramuscular depot injection that maintains mu receptor blockade for approximately one month per dose. It has no intrinsic agonist activity and no abuse potential. Its defining clinical requirement is complete opioid detoxification before initiation — a minimum of 7 to 10 days off short-acting opioids and longer for methadone. If administered to a patient with residual physical opioid dependence, it precipitates severe acute withdrawal. The monthly formulation removes the daily adherence decision, which improves compliance compared to oral naltrexone. A full mu agonist dispensed through licensed opioid treatment programs describes methadone. A partial mu agonist combined with naloxone for office-based treatment describes buprenorphine/naloxone. A peripherally restricted antagonist for gastrointestinal motility describes methylnaltrexone.

Correct Answer

B — Synthetic phenylpiperidine full mu opioid receptor agonist listed on the Beers Criteria, avoided in elderly patients and patients with renal impairment due to neuroexcitatory metabolite accumulation

Rationale

Meperidine is a synthetic phenylpiperidine full mu opioid receptor agonist. Its defining clinical liability is the accumulation of its metabolite normeperidine — a neuroexcitatory compound that lowers the seizure threshold, causing tremors, myoclonus, and seizures. Normeperidine is renally cleared, so accumulation is accelerated in patients with renal impairment. Meperidine is explicitly listed on the American Geriatrics Society Beers Criteria of potentially inappropriate medications in older adults, where reduced renal clearance magnifies the risk. A naturally occurring phenanthrene prodrug requiring cytochrome P450 2D6 conversion describes codeine. A semisynthetic phenanthrene preferred in renal impairment describes hydromorphone. A synthetic phenylheptylamine with N-methyl-D-aspartate antagonism describes methadone.

Correct Answer

A — Naturally occurring phenanthrene opioid prodrug contraindicated in children undergoing tonsillectomy or adenoidectomy and in breastfeeding mothers, due to risk of fatal toxicity in cytochrome P450 2D6 ultrarapid metabolizers

Rationale

Codeine is a naturally occurring phenanthrene alkaloid and an opioid prodrug — it depends on cytochrome P450 2D6-mediated conversion to morphine for its analgesic effect. Cytochrome P450 2D6 ultrarapid metabolizers convert codeine to morphine at an accelerated rate, generating morphine concentrations that can be fatal in vulnerable patients. This risk led the United States Food and Drug Administration to issue a black box warning against codeine use in children under age 12, in pediatric patients after tonsillectomy or adenoidectomy regardless of age, and in breastfeeding mothers. A synthetic phenylpiperidine on the Beers Criteria due to neuroexcitatory metabolite accumulation describes meperidine. A semisynthetic phenanthrene with high oral bioavailability describes oxycodone. A synthetic phenylpiperidine preferred in renal impairment describes fentanyl.

Correct Answer

D — Synthetic phenylpiperidine full mu opioid receptor agonist preferred over morphine in renal impairment due to less active metabolite accumulation, available in transdermal formulation

Rationale

Fentanyl is a synthetic phenylpiperidine full mu opioid receptor agonist. Unlike morphine, whose active metabolite morphine-6-glucuronide accumulates in renal failure and causes prolonged respiratory depression, fentanyl's metabolites are substantially less pharmacologically active, making it a preferred opioid in patients with significant renal impairment. Its high lipophilicity enables transdermal delivery through a patch formulation that releases drug continuously over 72 hours. A naturally occurring phenanthrene prodrug requiring hepatic conversion describes codeine. A synthetic phenylheptylamine with N-methyl-D-aspartate antagonism describes methadone. A semisynthetic phenanthrene producing an active glucuronide metabolite that accumulates in renal failure describes morphine.

Correct Answer

B — Strong opioids are titrated to effect with no fixed ceiling dose for cancer pain, continued alongside non-opioid analgesics and adjuvants, following the principles of by mouth, by the clock, and by the ladder

Rationale

Step 3 of the World Health Organization analgesic ladder introduces strong opioids — morphine, oxycodone, fentanyl, or hydromorphone — titrated to the dose that provides adequate pain control with acceptable adverse effects. For cancer pain, there is no fixed upper dose limit; the appropriate dose is whatever achieves the therapeutic goal. The guiding principles of the ladder — by mouth (oral route preferred when feasible), by the clock (around-the-clock dosing to maintain steady analgesia rather than waiting for pain to return), and by the ladder (stepwise escalation based on response) — continue to apply. Non-opioid analgesics and adjuvants are continued at Step 3 for their opioid-sparing contribution. The 90 milligram morphine equivalent ceiling applies to chronic non-cancer pain guidelines, not to cancer pain. Non-opioid analgesics are continued, not stopped. Oral strong opioids are bioavailable and are the preferred route when feasible.

Correct Answer

D — Methadone's long half-life allows once-daily dosing that maintains stable mu receptor occupancy, eliminating withdrawal symptoms and cravings without the peaks and troughs of short-acting illicit opioids

Rationale

Methadone maintenance works through stable pharmacological replacement: methadone's long half-life of 24 to 36 hours or longer allows once-daily oral dosing that maintains continuous mu receptor occupancy. This stable receptor activation eliminates the withdrawal symptoms and intense cravings that drive compulsive opioid use, and it removes the sharp peaks and troughs produced by short-acting illicit opioids. By satisfying the neuroadapted opioid system pharmacologically and predictably, methadone reduces drug-seeking behavior. Methadone is a full agonist, not an antagonist — it does not block mu receptors from illicit opioids. N-methyl-D-aspartate antagonism contributes to methadone's pharmacological profile but is not the mechanism of its anti-craving effects in maintenance therapy. Methadone is a full agonist with no ceiling effect — that characteristic belongs to buprenorphine.

Correct Answer

A — Fluconazole inhibits cytochrome P450 3A4, reducing methadone metabolism and raising plasma methadone levels, which prolongs the QTc interval through hERG potassium channel blockade

Rationale

Methadone prolongs cardiac ventricular repolarization by blocking hERG potassium channels, an effect that manifests as QTc prolongation on the electrocardiogram. The degree of QTc prolongation is dose-dependent and plasma concentration-dependent. Methadone is metabolized primarily by cytochrome P450 3A4. Fluconazole is a potent cytochrome P450 3A4 inhibitor — adding it to methadone therapy reduces methadone clearance, causing plasma methadone concentrations to rise above the intended steady state. Higher methadone concentrations produce greater hERG channel blockade and more pronounced QTc prolongation, increasing the risk of torsades de pointes and ventricular fibrillation. Electrocardiogram monitoring and dose adjustment are required when cytochrome P450 3A4 inhibitors are added to methadone. Fluconazole has no mu receptor activity in cardiac tissue. Fluconazole inhibits, not induces, cytochrome P450 3A4. Protein binding displacement is not the operative mechanism here.

Correct Answer

C — Morphine is hepatically metabolized to morphine-6-glucuronide, an active metabolite more potent than morphine itself that is renally excreted and accumulates in renal failure, causing prolonged respiratory depression

Rationale

Morphine undergoes hepatic glucuronidation to two major metabolites: morphine-6-glucuronide, which is pharmacologically active and more potent than the parent drug at mu receptors, and morphine-3-glucuronide, which has neuroexcitatory properties. Both metabolites are renally excreted. In patients with renal impairment, morphine-6-glucuronide accumulates with repeated dosing, producing respiratory depression that is prolonged relative to the parent drug's expected duration. This metabolite-driven toxicity can develop insidiously — a patient may tolerate initial doses well but develop respiratory compromise as metabolite levels build over time. Preferred alternatives in renal impairment include fentanyl and hydromorphone, whose metabolites are less pharmacologically active. Morphine itself is primarily hepatically cleared, not renally excreted unchanged. Normeperidine is a metabolite of meperidine, not morphine. Morphine does not reduce glomerular filtration rate through kappa receptors.

Correct Answer

B — Meperidine is metabolized to normeperidine, a neuroexcitatory compound that accumulates in elderly patients due to reduced renal clearance, lowering the seizure threshold and causing tremors, myoclonus, and seizures

Rationale

Normeperidine is the major metabolite of meperidine and has neuroexcitatory properties distinct from meperidine's opioid effects. It lowers the seizure threshold and produces a progression from mild anxiety and tremors through myoclonus to frank seizures as it accumulates. Because normeperidine is renally excreted, elderly patients — who have reduced renal clearance even with apparently normal creatinine levels due to reduced muscle mass — are at amplified risk. Unlike opioid-induced respiratory depression, normeperidine neurotoxicity is not reversed by naloxone. The American Geriatrics Society Beers Criteria explicitly lists meperidine as a potentially inappropriate medication in older adults for this reason. Meperidine does have serotonin reuptake inhibitor activity, which is a real concern with monoamine oxidase inhibitors, but this is not the basis for the Beers Criteria listing. Kappa receptor dysphoria is an effect of mixed agonist-antagonist drugs, not meperidine.

Correct Answer

A — Methadone or buprenorphine maintenance is recommended over abstinence because abrupt opioid withdrawal in pregnancy risks fetal distress, preterm labor, and miscarriage — neonatal opioid withdrawal syndrome is an expected and manageable outcome of maintenance therapy

Rationale

Current guidelines from major obstetric and addiction medicine organizations recommend opioid agonist maintenance therapy — methadone or buprenorphine — for pregnant women with opioid use disorder rather than abstinence. Abrupt opioid withdrawal during pregnancy can precipitate fetal distress, placental abruption, preterm labor, and pregnancy loss. The goal is a stable maternal opioid level that prevents these obstetric risks. Neonatal opioid withdrawal syndrome — onset 24 to 72 hours after delivery — is an expected consequence of fetal opioid exposure and is a manageable neonatal condition that does not justify withholding maternal treatment. Medically supervised detoxification during pregnancy carries fetal risk and high relapse rates. Extended-release naltrexone requires full detoxification before initiation and has limited data in pregnancy — it is not the preferred first-line agent. Pharmacological treatment with opioid agonist maintenance is the evidence-based standard.

Correct Answer

D — A minimum of 7 to 10 days off short-acting opioids is required to ensure physical dependence has resolved sufficiently — if residual dependence is present, naltrexone's mu receptor blockade precipitates severe acute withdrawal

Rationale

Extended-release naltrexone is a pure opioid antagonist that blocks mu receptors for approximately one month after each injection. If it is administered while the patient retains physical opioid dependence, the abrupt and sustained mu receptor blockade precipitates severe acute withdrawal that cannot be easily reversed because of naltrexone's long duration of action. A minimum of 7 to 10 days off short-acting opioids — and longer for methadone, which has a much longer half-life — is required for physical dependence to resolve enough that the first naltrexone injection is safe. A naloxone challenge test can be performed before the first injection to confirm readiness. The detoxification period itself carries elevated relapse and overdose risk, which is one of the practical challenges of initiating extended-release naltrexone compared to buprenorphine. A 24-hour period is insufficient. There is no gradual mechanism that avoids precipitated withdrawal — the effect of naltrexone at mu receptors is immediate and complete.

Correct Answer

C — A 58-year-old man on high-dose extended-release oxycodone who also takes alprazolam for anxiety and has a prior emergency department visit for opioid overdose

Rationale

Opioid stewardship guidelines recommend co-prescribing naloxone for patients at elevated overdose risk. This patient has three major risk factors that together represent the strongest indication: high opioid dose (extended-release oxycodone at high dose increases respiratory depression risk), concurrent benzodiazepine use (alprazolam combined with opioids produces additive central nervous system and respiratory depression — one of the most dangerous drug combinations in prescribing), and prior overdose history (the strongest predictor of future overdose risk). Current guidelines from multiple professional organizations endorse naloxone co-prescribing for patients with any of these risk factors, and this patient has all three. A patient on low-dose codeine with no risk factors has a low absolute risk. A patient requesting opioids for the first time who has not yet started them does not yet have an indication for naloxone co-prescribing. A patient on tramadol with no concurrent risk factors has lower risk than the patient described in option C.

Correct Answer

D — Acute deterioration in renal function has reduced excretion of morphine-6-glucuronide, allowing this active metabolite to accumulate and produce respiratory depression beyond what the morphine dose alone would cause

Rationale

This patient was tolerating his morphine dose at baseline renal function, but acute kidney injury during his hospitalization has reduced the clearance of morphine-6-glucuronide — the pharmacologically active glucuronide metabolite of morphine that is more potent than the parent drug at mu receptors. As morphine-6-glucuronide accumulates, its contribution to mu receptor activation increases, producing opioid effects that exceed what would be expected from the morphine dose alone. This mechanism explains why a previously well-tolerated morphine regimen can become acutely dangerous when renal function deteriorates. Management includes discontinuing or dose-reducing morphine, providing respiratory support, and considering a switch to fentanyl or hydromorphone once the acute situation is stabilized. General anesthesia does not produce a delayed two-day interaction with morphine through the mechanism described. Pain-mediated arousal does contribute to opioid tolerance, but metabolite accumulation is the operative mechanism in this case. Morphine itself is hepatically cleared; the kidney is responsible for metabolite excretion.

Correct Answer

B — Normeperidine, the neuroexcitatory metabolite of meperidine, accumulated over two days due to reduced renal clearance, producing progressive central nervous system excitation culminating in seizure

Rationale

This case illustrates the clinical consequence that led to meperidine's listing on the Beers Criteria. Meperidine is metabolized hepatically to normeperidine, a neuroexcitatory metabolite with no meaningful analgesic activity but substantial neurotoxic potential. Normeperidine is renally excreted and accumulates progressively with repeated dosing in patients with renal impairment. Elderly patients are doubly vulnerable: reduced renal clearance from age-related nephron loss accelerates normeperidine accumulation, and reduced cognitive and neurological reserve lowers the threshold for seizure. The two-day time course — normal initial response followed by progressive tremors, agitation, and finally seizure — is characteristic of metabolite accumulation rather than direct drug toxicity. Normeperidine-induced seizures are not reversed by naloxone because they are not opioid receptor-mediated. Meperidine's neuroexcitatory effects are metabolite-driven, not directly from mu receptor activation. Meperidine's serotonin activity is a risk with monoamine oxidase inhibitors but does not produce progressive accumulation-type toxicity. Meperidine itself is metabolized by multiple hepatic pathways, not selectively by cytochrome P450 2D6.

Correct Answer

A — Start methadone or buprenorphine maintenance — opioid agonist maintenance therapy is the recommended standard because abrupt withdrawal in pregnancy risks fetal distress, placental abruption, preterm labor, and pregnancy loss

Rationale

Opioid agonist maintenance therapy with methadone or buprenorphine is the evidence-based standard of care for opioid use disorder in pregnancy. The patient's goal of abstinence, while understandable, is medically inadvisable: abrupt opioid withdrawal causes sympathetic activation, fetal catecholamine surges, and uterine contractions that can lead to fetal distress, placental abruption, preterm labor, and pregnancy loss. The goal of treatment is stable maternal opioid levels, not abstinence. Neonatal opioid withdrawal syndrome is an expected consequence of maintenance therapy and is a manageable neonatal condition — its occurrence is not a reason to withhold maternal treatment. Medically supervised withdrawal during pregnancy carries high relapse rates and has not been shown to improve neonatal outcomes compared to maintenance. Extended-release naltrexone requires full opioid detoxification before initiation — which itself carries the fetal risks described — and has limited safety data in pregnancy. Behavioral counseling alone, without pharmacological stabilization, leaves the patient and fetus at continued risk from illicit opioid use.

Correct Answer

C — Four weeks of mu receptor blockade by naltrexone reversed his opioid tolerance — when naltrexone cleared and he resumed heroin at his previous dose, his opioid-naive mu receptor sensitivity produced severe respiratory depression

Rationale

This case demonstrates the most dangerous consequence of extended-release naltrexone treatment: loss of tolerance during the blockade period. Opioid tolerance is maintained by ongoing mu receptor activation. During four weeks of complete mu receptor blockade by naltrexone, the patient's tolerance reversed — his mu receptors returned toward their baseline pre-addiction sensitivity without the chronic opioid stimulation that had maintained tolerance. When naltrexone cleared after the missed second injection and the patient resumed heroin at his previous dose, his now opioid-naive receptor system responded to that dose as an opioid-naive individual would — with severe respiratory depression. This mechanism accounts for the elevated overdose fatality rates observed in the period after naltrexone treatment ends or is interrupted. Patients must be counseled explicitly that their opioid tolerance is lost during naltrexone treatment and that their previous opioid doses are no longer safe if they relapse. Naltrexone is a pure antagonist — it does not produce additive respiratory depression with opioids. Naltrexone does not affect cytochrome P450 enzymes. Missing a dose does not cause rebound upregulation of mu receptors.