Opioid Study Guide

📖 Core Concepts Opioid: Any natural, semi‑synthetic, or synthetic substance that binds opioid receptors (μ, κ, δ) to produce morphine‑like effects. Opiate: Historically, only natural alkaloids from Papaver somniferum (e.g., morphine, codeine). Receptor subtypes μ‑receptors → analgesia, euphoria, respiratory depression, constipation, physical dependence. κ‑receptors → spinal analgesia, sedation, dysphoria, diuresis. δ‑receptors → analgesia & mood modulation (clinical role less defined). Mechanism: Opioids activate G‑protein‑coupled receptors → inhibit GABA release → ↓ neuronal excitability → analgesia & other effects. Tolerance vs. Dependence Tolerance: neuroadaptation requiring higher doses for same effect; rapid for analgesia, slow for respiratory depression. Physical dependence: withdrawal syndrome when drug is stopped abruptly. Addiction (Opioid Use Disorder): compulsive drug‑seeking despite harm; distinct from tolerance/dependence. Opioid‑Induced Hyperalgesia (OIH): paradoxical increased pain sensitivity after high‑dose or rapid escalation. 📌 Must Remember μ‑receptor affinity → higher analgesic potency and higher respiratory‑depression risk. Respiratory depression is the leading cause of fatal overdose; risk ↑ with benzodiazepines, alcohol, high IV dose. Constipation does NOT develop tolerance → treat prophylactically (fiber, fluids, laxatives, peripherally selective antagonists). Naloxone: short‑acting competitive antagonist; may need repeat dosing/infusion for long‑acting opioids (e.g., methadone). CDC 2016 guideline: limit chronic therapy to ≤90 morphine‑milligram equivalents (MME) per day when possible. Physical dependence → withdrawal; withdrawal severity correlates with opioid half‑life (heroin/morphine fast, methadone slow). Opioid rotation uses equianalgesic tables to reduce tolerance/OIH and manage side‑effects. 🔄 Key Processes Assessing Opioid Need Evaluate pain intensity, functional impairment, psychosocial risks. Prefer non‑opioid therapies first; consider opioids only for moderate‑severe acute, cancer, or refractory chronic pain. Initiation & Titration Start with lowest effective dose of a short‑acting opioid. Increase dose gradually while monitoring pain relief and adverse effects. Monitoring & Follow‑up Schedule regular visits (every 1–4 weeks initially). Use urine drug testing, PDMP checks, and validated risk tools. Managing Constipation Begin fiber + ≥1.5 L fluid + regular activity. Add osmotic laxative (e.g., polyethylene glycol); if refractory, add peripherally selective μ‑antagonist (e.g., methylnaltrexone). Opioid Rotation Convert current opioid dose → equianalgesic dose of new opioid (apply 25–30 % dose reduction for incomplete cross‑tolerance). Re‑titrate to achieve analgesia with fewer side effects. Overdose Reversal Administer naloxone 0.4–2 mg IV/IM; repeat every 2–3 min if respiratory depression persists. For long‑acting opioid overdose, consider nalmefene or repeat naloxone infusion. 🔍 Key Comparisons Natural vs. Semi‑synthetic vs. Synthetic Natural: morphine, codeine – directly from opium. Semi‑synthetic: hydrocodone, oxycodone, fentanyl – chemically modified from natural opiates. Synthetic: fentanyl, methadone, tramadol – fully lab‑manufactured. μ‑1 vs. μ‑2 receptors μ‑1 → supraspinal analgesia, euphoria. μ‑2 → respiratory depression, physical dependence. Naloxone vs. Naltrexone Naloxone: rapid onset, short half‑life → emergency reversal. Naltrexone: oral, long half‑life → used for relapse prevention, not acute overdose. ⚠️ Common Misunderstandings “Tolerance means you can’t become addicted.” → Tolerance reduces effect, but addiction risk persists, especially with misuse. “Constipation will improve with higher doses.” → No tolerance develops; constipation remains constant. “Opioids are safe for chronic non‑cancer pain.” → Guidelines recommend caution; first‑line non‑opioid agents preferred. “Naloxone can be given once and forget it.” → Short half‑life may require repeat dosing for long‑acting opioids. 🧠 Mental Models / Intuition “Receptor affinity = effect intensity” – Visualize each opioid as a key fitting more tightly into μ‑locks → stronger analgesia + more side effects. “Tolerance ladder” – Analgesia climbs quickly, respiratory depression climbs slowly → early dosing changes impact pain relief far more than safety. “Opioid‑risk triangle” – Combine three factors (high dose, concurrent depressant, opioid‑naïve) → exponential overdose risk. 🚩 Exceptions & Edge Cases Partial agonists (e.g., buprenorphine): high μ affinity, low intrinsic activity → ceiling effect for respiratory depression, useful in OUD treatment. Peripheral antagonists (methylnaltrexone, naloxegol): do not cross BBB → relieve constipation without reversing analgesia. Long‑acting opioid overdose: naloxone may wear off → consider continuous infusion or longer‑acting antagonist (nalmefene). 📍 When to Use Which Acute severe pain → short‑acting oral/IV opioid (e.g., morphine, oxycodone). Chronic cancer pain → long‑acting oral μ‑agonist (e.g., sustained‑release morphine) ± breakthrough dosing. Opioid‑induced constipation → start osmotic laxative; if refractory → peripherally selective μ‑antagonist. Overdose reversal → naloxone (IV/IM); if opioid with long half‑life → repeat or switch to nalmefene. Opioid Use Disorder → buprenorphine‑naloxone (partial agonist) or methadone (full agonist) for maintenance. 👀 Patterns to Recognize Rapid escalation + high dose → look for OIH (pain worsening despite higher doses). Early nausea/vomiting → anticipate tolerance within 7–10 days; consider antiemetic prophylaxis. Sedation within first week → expect tolerance by day 5–7; reassess need for dose reduction. Concurrent benzodiazepine prescription → flag high overdose risk. 🗂️ Exam Traps “Opioids cause organ toxicity (e.g., kidney damage).” – Wrong; they cause functional GI effects, not direct organ injury. “All opioids cause the same level of respiratory depression.” – Incorrect; μ‑2 receptor affinity and dose dictate risk. “Tolerance develops equally to all side effects.” – False; tolerance is rapid for analgesia, slow for respiratory depression, and absent for constipation. “Naloxone is contraindicated in opioid‑dependent patients.” – Misleading; it reverses overdose regardless of dependence, though precipitated withdrawal may occur. “Opioid rotation always requires a 50 % dose reduction.” – Oversimplified; standard practice is 25–30 % reduction for incomplete cross‑tolerance. --- Use this guide for rapid recall before exams—focus on the bolded “must‑remember” facts and the decision‑rules in “When to Use Which.” Good luck!