Opioid pharmacology is essential for the neurologist treating acute severe pain (post-surgical, trauma, fracture), cancer pain, palliative care, opioid-induced complications (neurotoxicity, withdrawal, intoxication), and opioid use disorder. The opioid epidemic has transformed prescribing practices, but opioids remain critical tools in selected scenarios. This page covers opioid pharmacology, prescribing principles, opioid use disorder, opioid-induced complications, and the neurologist’s role in opioid stewardship.

Opioid Receptor Pharmacology

Receptor Subtypes

  • μ (mu): classical analgesia; respiratory depression; euphoria; constipation; physical dependence.
  • κ (kappa): spinal analgesia; dysphoria; psychotomimetic.
  • δ (delta): emotional regulation; less clinical relevance.
  • All G-protein coupled.

Distribution

  • Central: periaqueductal gray, brainstem (respiratory centers, area postrema), spinal cord dorsal horn.
  • Peripheral: GI tract, peripheral nerves.

Major Opioids

Short-Acting

  • Morphine: gold standard; oral, IV, PR; active metabolite (M3G, M6G — morphine glucuronides).
  • Oxycodone: oral; combinations with acetaminophen.
  • Hydrocodone: combination with acetaminophen.
  • Hydromorphone: more potent; less histamine release than morphine.
  • Fentanyl: IV, transmucosal, transdermal patch; potent; rapid onset; short duration (acute).

Long-Acting / Extended-Release

  • MS Contin (morphine ER), OxyContin (oxycodone ER), fentanyl patch (72-96 hour), methadone, buprenorphine.
  • Use for chronic pain.
  • Do NOT crush ER formulations (immediate release risk).

Specific Opioids

  • Methadone: long half-life (variable); μ agonist + NMDA antagonism + 5-HT/NE reuptake inhibition; complex; used for chronic pain (titrate carefully) and opioid use disorder maintenance; QT prolongation.
  • Buprenorphine: partial μ agonist + κ antagonist; ceiling effect on respiratory depression; for opioid use disorder; sublingual or transdermal patch for chronic pain.
  • Tramadol: weak μ agonist + SNRI; seizure threshold lowering; serotonin syndrome with SSRIs.
  • Tapentadol: μ agonist + NET inhibition; less constipation; less itching.
  • Codeine: CYP2D6 prodrug → morphine; PMs (poor metabolizers) get inadequate analgesia; UMs (ultrarapid) at risk of opioid toxicity, especially in breastfeeding mothers and children.
  • Meperidine (Demerol): largely abandoned (toxic metabolite normeperidine causes seizures); avoid with MAOIs.

Equianalgesic Conversion

Approximate oral morphine equivalents (variable):

  • Oxycodone 20 mg ≈ Morphine 30 mg PO.
  • Hydromorphone 7.5 mg ≈ Morphine 30 mg PO.
  • Hydrocodone 30 mg ≈ Morphine 30 mg PO.
  • Methadone: complex; not linear; conservative conversion (e.g., 1:1 → 1:4 depending on dose); always use lower estimate.
  • Fentanyl 25 μg/hr patch ≈ Morphine 60-100 mg/day PO.

Prescribing Principles

CDC Guideline Principles (2022 Update)

  • Non-opioid first for chronic pain.
  • If opioid: start low, go slow.
  • Avoid combining opioid + benzodiazepine (respiratory depression).
  • Risk-benefit assessment at start and at increases.
  • 3 days usually sufficient for acute pain (rarely >7).
  • Avoid prescribing >50 MME/day for new chronic pain (CDC threshold).
  • Caution >90 MME/day.
  • Treat opioid use disorder as disease, not moral failing.

For Specific Conditions

  • Acute pain: opioids appropriate; 3-5 days usually.
  • Cancer pain: opioids appropriate; long-term acceptable.
  • End-of-life / palliative: appropriate; titrate to symptoms.
  • Chronic non-cancer pain: limited evidence; risk-benefit unfavorable for most.
  • Neuropathic pain: opioids less effective than for nociceptive pain.
  • Migraine: AVOID; cause MOH.
  • Fibromyalgia: AVOID.

Opioid Adverse Effects

Common

  • Constipation (universal; treat prophylactically).
  • Nausea (often improves; antiemetics).
  • Sedation, mental fog.
  • Itching (histamine release; morphine particularly).
  • Urinary retention.

Serious

  • Respiratory depression: dose-related; deadly with rapid titration, combinations.
  • Hypotension.
  • Bradycardia.
  • Drug-drug interactions (especially CYP3A4 substrates with methadone, fentanyl).

Chronic

  • Tolerance, physical dependence, addiction.
  • Hyperalgesia (paradoxical pain sensitization).
  • Hypogonadism.
  • Immunosuppression.
  • Constipation (chronic).
  • Sleep apnea.

Neurologic Specific

  • Myoclonus (especially morphine; M3G metabolite).
  • Seizures (meperidine, tramadol).
  • Cognitive impairment.
  • Acute opioid-induced encephalopathy.

Opioid Overdose

Recognition

  • Respiratory depression (rate <12); pinpoint pupils; obtundation; cyanosis.

Treatment

  • Naloxone (Narcan): μ antagonist.
  • 0.4-2 mg IV/IM/intranasal; repeat q2 min; up to 10 mg.
  • Short half-life (60-90 min) vs longer opioids (especially methadone, sustained release) — may need infusion or repeat dosing.
  • Precipitates withdrawal in chronic users (autonomic crisis but generally safe).
  • Nalmefene (Opvee): long-acting nasal alternative (FDA-approved 2023); 48-hour duration.
  • Naloxone widely available without prescription (good Samaritan laws); essential for opioid prescriber/community.

Opioid Withdrawal

  • Symptoms: anxiety, restlessness, sweating, lacrimation, rhinorrhea, mydriasis, myalgia, abdominal cramps, nausea, vomiting, diarrhea, tachycardia, hypertension.
  • Generally NOT life-threatening (unlike alcohol/benzodiazepine).
  • COWS scale for assessment.

Management

  • Clonidine: α2 agonist; reduces autonomic symptoms; 0.1 mg BID-TID.
  • Lofexidine: α2 agonist; FDA-approved for withdrawal.
  • Buprenorphine: rapid relief of withdrawal symptoms; can transition to maintenance.
  • Methadone: replacement; for inpatient setting.
  • Anti-emetics, anti-diarrheals, NSAIDs, sleep aids.
  • Neonatal abstinence syndrome: morphine, methadone, buprenorphine for opioid-exposed neonates.

Opioid Use Disorder Treatment

Medication-Assisted Treatment (MAT)

  • Buprenorphine (suboxone): partial agonist + naloxone combination (reduces IV abuse); X-waiver no longer required (US 2023); office-based.
  • Methadone: full agonist; specialized clinic; daily dosing.
  • Naltrexone: μ antagonist; oral or long-acting IM (Vivitrol monthly); requires opioid abstinence before starting (precipitates withdrawal).
  • MAT shown to reduce mortality, improve outcomes vs detoxification alone.

Behavioral / Support

  • Counseling; AA/NA; sober living.
  • Mental health treatment for comorbidities.
  • Harm reduction (naloxone availability, safer use supplies).

Opioid-Induced Constipation

  • Treat prophylactically.
  • Senna, polyethylene glycol, lactulose.
  • Peripheral μ antagonists for opioid-induced constipation (do not affect analgesia):
    • Methylnaltrexone (Relistor): SC.
    • Naloxegol (Movantik): oral.
    • Naldemedine (Symproic): oral.

Opioids in Special Populations

Renal Impairment

  • Morphine: M6G accumulates; toxicity risk; AVOID.
  • Codeine: CYP2D6 variable; avoid.
  • Methadone: relatively safe in CKD (hepatic clearance).
  • Fentanyl: hepatic clearance; relatively safe.
  • Hydromorphone: alternative.

Hepatic Impairment

  • Most opioids metabolized hepatically; reduce dose, extend interval.
  • Methadone: tricky; significant pharmacokinetic changes.

Elderly

  • Start low, go slow.
  • Increased CNS sensitivity.
  • Higher fall risk.

Pregnancy

  • Acute pain: morphine acceptable.
  • Chronic opioid in pregnancy: NAS (neonatal abstinence syndrome) risk.
  • OUD in pregnancy: buprenorphine or methadone (not detox during pregnancy).

Drug Interactions

  • Benzodiazepines: respiratory depression; FDA black box warning for combination.
  • Alcohol: respiratory depression.
  • Methadone CYP3A4 substrate: many drug interactions.
  • Tramadol + SSRI: serotonin syndrome.
  • Methadone + QT-prolonging drugs: torsade risk.
  • Codeine in CYP2D6 ultra-rapid metabolizers: opioid toxicity (especially infants of breastfeeding mothers).

Special Neurology Considerations

  • Migraine: opioids cause MOH and are ineffective; AVOID.
  • Tramadol: lowers seizure threshold; avoid in epileptic patients.
  • Methadone: QT prolongation; baseline and follow-up ECG.
  • Fentanyl patch: fever increases absorption (heat warning).
  • Neurologic complications of OUD: stroke, encephalopathy, peripheral neuropathy.

🔍 Did You Know?

Codeine-induced toxicity in CYP2D6 ultra-rapid metabolizers, particularly in breastfeeding mothers passing morphine to infants, has been one of the most important pharmacogenomic safety lessons in modern medicine. Codeine is a prodrug — it requires CYP2D6 to convert to morphine for analgesic effect. CYP2D6 ultra-rapid metabolizers (UMs) — approximately 1-30% of populations depending on ancestry — convert codeine to morphine far more efficiently than normal, producing potentially toxic morphine levels. Pediatric deaths and severe morbidity from codeine, particularly in breastfeeding mothers of CYP2D6 UMs whose breast milk contained dangerous morphine levels, led to: FDA contraindication of codeine in children under 12 and in nursing mothers, and broader awareness of CYP2D6 testing for opioid prescribing. This has driven adoption of alternative analgesics in pediatrics (acetaminophen, ibuprofen, or non-codeine opioids when needed) and counseling for nursing mothers requiring opioid analgesia. The lesson generalizes: pharmacogenomic variation in drug metabolism can transform “safe” drugs into dangerous ones in specific genotypes, and prescribing must consider population-level pharmacogenomic patterns. The same principle applies to clopidogrel (CYP2C19), tramadol (CYP2D6), warfarin (CYP2C9 + VKORC1), and many other drugs. For practicing neurologists, the take-home: question whether the patient is a “fast metabolizer” if codeine or tramadol produce unexpected effects, and consider alternative agents in vulnerable populations.

Pitfalls and Pearls

  • Opioid receptors: μ (analgesia + respiratory depression), κ (dysphoria, spinal), δ.
  • CDC guidelines: limit acute prescriptions, avoid combination with benzodiazepines, treat OUD.
  • Avoid in migraine: cause MOH; less effective.
  • Methadone: complex; long half-life; QT prolongation; CYP3A4 interactions.
  • Buprenorphine: partial agonist; ceiling on respiratory depression; OUD maintenance.
  • Tramadol: SNRI component; seizures; serotonin syndrome with SSRI.
  • Codeine: CYP2D6 prodrug; AVOID in children, breastfeeding mothers.
  • Meperidine: largely abandoned (normeperidine seizures, MAOI interaction).
  • Morphine in CKD: M6G accumulates; AVOID.
  • Naloxone: μ antagonist; available without prescription; 0.4-2 mg IV/IM/IN.
  • Naloxone half-life: shorter than methadone/ER formulations; may need infusion or repeat.
  • Withdrawal: clonidine, lofexidine, buprenorphine, methadone.
  • MAT: buprenorphine, methadone, naltrexone — reduces mortality.
  • Buprenorphine X-waiver: no longer required (US 2023).
  • Opioid-induced constipation: prophylactic; methylnaltrexone, naloxegol if standard fails.
  • Benzodiazepine + opioid: respiratory depression; black box.

References

  1. Dowell D, Ragan KR, Jones CM, Baldwin GT, Chou R. CDC Clinical Practice Guideline for Prescribing Opioids for Pain—United States, 2022. MMWR Recomm Rep. 2022;71(3):1-95.
  2. Volkow ND, Collins FS. The role of science in addressing the opioid crisis. N Engl J Med. 2017;377(4):391-394.
  3. Crews KR, Gaedigk A, Dunnenberger HM, et al. CPIC guideline for cytochrome P450 2D6 genotype and codeine therapy. Clin Pharmacol Ther. 2014;95(4):376-382.
  4. Mattick RP, Breen C, Kimber J, Davoli M. Buprenorphine maintenance versus placebo or methadone maintenance for opioid dependence. Cochrane Database Syst Rev. 2014;(2):CD002207.
  5. Brunton LL, Hilal-Dandan R, Knollmann BC, eds. Goodman & Gilman’s The Pharmacological Basis of Therapeutics. 14th ed. McGraw-Hill; 2023.