Non-opioid Analgesics

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Non-opioids: Intro & MOA - The Pain Tamers

  • Definition: Diverse drug group for mild-moderate pain & fever; some possess significant anti-inflammatory effects (e.g., NSAIDs). Not structurally related to opioids, generally fewer side effects like dependence.
  • Key Classes:
    • NSAIDs (e.g., Ibuprofen, Diclofenac)
    • Paracetamol (Acetaminophen) - analgesic, antipyretic, weak anti-inflammatory.
    • Others (e.g., Nefopam - centrally acting, non-opioid).
  • NSAID MOA - Primary Action:
    • Inhibit cyclooxygenase (COX) enzymes (isoforms COX-1 & COX-2).
    • This blocks conversion of arachidonic acid (released by phospholipase A2) to prostaglandins (PGs), prostacyclin (PGI2), & thromboxane A2 (TXA2).
    • PGs: Key mediators of pain, inflammation, fever.
    • 📌 Mnemonic: "NSAIDs knock out COX, PGs can't box!"

⭐ Most NSAIDs non-selectively inhibit COX-1 & COX-2, yielding therapeutic effects but also side effects (e.g., gastric issues via COX-1 inhibition).

Arachidonic Acid Pathway and Ibuprofen Mechanism

NSAIDs: Types & ADRs - COX & Effects

  • COX Enzymes & Effects:
    • COX-1 (Constitutive): GI protection (PGE2/PGI2), Platelet aggregation (TXA2), Renal function.
    • COX-2 (Inducible/Constitutive): Inflammation, Pain, Fever. Also renal function, PGI2 (vasodilation, anti-platelet) in endothelium.

COX/LOX pathway and NSAID/Coxib inhibition

  • NSAID Classes & Comparative ADRs:
ClassExamplesKey ADRs (Relative Risk)
Non-selectiveIbuprofen, Aspirin, DiclofenacGI: High (ulcers, bleed). Renal: AKI. CV: ↑BP, thrombotic risk (Aspirin cardioprotective at low dose). Antiplatelet effect.
Preferential COX-2Nimesulide, MeloxicamGI: Moderate. Renal/CV: Similar to non-selective.
Selective COX-2 (Coxibs)Celecoxib, EtoricoxibGI: Lower. Renal: Similar. CV: ↑↑ Thrombotic risk (MI, stroke). No antiplatelet effect. ⚠️ Sulfa allergy (Celecoxib).
*   **GI:** ↓Protective PGs.
*   **Renal:** ↓PGs → ↓RBF, Na+/H2O retention.
*   **CV (Coxibs/Non-Aspirin NSAIDs):** Unopposed TXA2 or ↓PGI2 → prothrombotic.
*   **AERD:** Leukotriene overproduction.

⭐ Aspirin: Irreversible COX-1 inhibition in platelets (antiplatelet effect for 7-10 days). Low dose (75-150 mg) for cardioprotection.

📌 COX-1 = Gut, Renal, Platelets (GRP). COX-2 = Inflammation, Pain, Fever (IPF).

Paracetamol: The Go-To - Beyond NSAIDs

  • Mechanism: Weak COX inhibitor (mainly central); possible effects on COX-3, serotonergic pathways. Analgesic, antipyretic.
    • ⚠️ No significant anti-inflammatory effect.
  • Dosing:
    • Adults: 0.5-1g QDS (max 4g/day).
    • Pediatrics: 10-15 mg/kg/dose.
  • Hepatotoxicity:
    • Toxic metabolite: NAPQI (N-acetyl-p-benzoquinone imine) via CYP450 (mainly CYP2E1).
    • Overdose depletes glutathione → NAPQI damages hepatocytes.
    • Toxic dose (acute): >150 mg/kg or >7.5-10g.
    • Risk factors: Chronic alcohol, malnutrition, enzyme inducers (e.g., rifampicin, isoniazid).
  • Antidote: N-acetylcysteine (NAC).
    • Replenishes glutathione, directly detoxifies NAPQI.
    • Best if given within 8-10 hours post-ingestion.

⭐ Rumack-Matthew nomogram guides NAC therapy based on serum paracetamol levels vs. time post-ingestion.

Paracetamol metabolism pathway and NAPQI detoxification

Paracetamol Poisoning Management Flowchart:

Clinical Use: Smart Choices - Pick Wisely

Choose non-opioids by pain type, severity, patient risks. Paracetamol is cornerstone if NSAIDs risky.

  • Paracetamol: Max dose 4g/day (adults). In severe hepatic impairment, limit to <2g/day.
  • NSAIDs: Use lowest effective dose, shortest duration. ⚠️ AVOID if GFR <30 mL/min, active PUD, or 3rd trimester pregnancy.
  • ⚠️ Interactions: NSAIDs with anticoagulants (↑bleeding); +ACEi/ARBs/diuretics (↑renal toxicity).

⭐ For osteoarthritis in elderly, paracetamol is first-line over NSAIDs due to better safety, despite weaker anti-inflammatory effects.

High‑Yield Points - ⚡ Biggest Takeaways

  • Paracetamol: Central COX inhibition; hepatotoxicity (NAPQI) reversed by N-acetylcysteine; max 4g/day.
  • NSAIDs: COX-1 & COX-2 inhibition; risks: GIT ulcers, renal impairment, bronchospasm.
  • Selective COX-2 inhibitors: ↓ GIT side effects, but ↑ cardiovascular thrombotic risk (e.g., Etoricoxib).
  • Nefopam: Centrally acting non-opioid; inhibits monoamine reuptake; causes tachycardia, sweating.
  • Low-dose Ketamine: NMDA antagonist; potent analgesic for neuropathic pain, opioid-sparing.
  • Gabapentinoids: Target α2δ Ca2+ channel subunit; first-line for neuropathic pain.

Practice Questions: Non-opioid Analgesics

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Which gene is responsible for the production of COX type 3?

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Flashcards: Non-opioid Analgesics

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_____ used for pain relief is based on the gate control theory of pain

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_____ used for pain relief is based on the gate control theory of pain

Transcutaneous electrical nerve stimulation (TENS)

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