Acetaminophen (Paracetamol)

Acetaminophen (Paracetamol): Basics - The Gentle Giant?

• AKA: Acetaminophen (APAP), Paracetamol.
• Key Actions: Potent analgesic (pain reliever) and antipyretic (fever reducer).
• Anti-inflammatory: Weak peripheral anti-inflammatory activity, distinguishing it from NSAIDs.
• **Proposed MOA (complex & primarily central):
  • Inhibition of cyclooxygenase (COX) in the CNS, possibly a specific isoform like COX-3.
  • Active metabolite AM404 (N-arachidonoylphenolamine) formed in the brain; inhibits anandamide reuptake, enhancing endocannabinoid tone.
  • Modulation of serotonergic descending inhibitory pathways.

⭐ Acetaminophen primarily exerts its analgesic and antipyretic effects centrally with minimal peripheral anti-inflammatory activity.

Acetaminophen (Paracetamol): Para's Journey - ADME Adventure

• Absorption: Rapid & complete post-oral administration.
• Distribution: Widely distributed; low plasma protein binding.
• Metabolism: Primarily hepatic. 📌 Metabolism: Glucuronidation & Sulfation (main). CYP2E1 $\rightarrow$ NAPQI (toxic), needs Glutathione.
  • Major pathways (Therapeutic doses): Glucuronidation (60%), Sulfation (30%). These form non-toxic conjugates.
  • Minor pathway: Oxidation by CYP450 (mainly CYP2E1, <10%) to form $N$-acetyl-$p$-benzoquinone imine (NAPQI), a highly reactive toxic metabolite.
  • Detoxification: NAPQI is rapidly conjugated with glutathione to form non-toxic mercapturic acid derivatives.
• Excretion: Predominantly renal, as glucuronide and sulfate conjugates.

⭐ The toxic metabolite NAPQI (N-acetyl-p-benzoquinone imine) is primarily formed via the CYP2E1 enzyme system when glucuronidation and sulfation pathways are saturated, e.g., in overdose.

Acetaminophen (Paracetamol): Healing Hands - Uses & Doses

• Indications:
  • Mild to moderate pain (e.g., headache, musculoskeletal pain).
  • Fever (effective antipyretic).
  • Preferred analgesic/antipyretic in:
    • Children.
    • Pregnancy (use with caution, short-term if possible).
    • Patients with peptic ulcer disease.
    • Individuals with aspirin-sensitive asthma.
• Dosage Forms:
  • Oral: Tablets, capsules, syrups, suspensions (most common route).
  • Rectal: Suppositories (useful if oral route is compromised).
  • Intravenous (IV): For rapid analgesia/antipyresis in clinical settings.
• Dosing:
  • Adults: 325-650 mg every 4-6 hours. Maximum daily dose: 4g/day.
  • Pediatrics: 10-15 mg/kg/dose every 4-6 hours. Maximum: 75 mg/kg/day or 4g/day.

⭐ Acetaminophen is a first-line analgesic/antipyretic for children and is generally considered safe in pregnancy, though prolonged use should be discussed with a doctor.

Acetaminophen (Paracetamol): Danger Zone - Toxicity & Antidote

• Toxicity: Hepatotoxicity due to NAPQI (N-acetyl-p-benzoquinone imine) when glutathione is depleted.
• Toxic Dose (Single Ingestion): Children >150 mg/kg; Adults >7.5-10 g.
• Risk Factors: Chronic alcohol, malnutrition, enzyme-inducing drugs (e.g., rifampicin, phenytoin).
• Clinical Stages (I-IV):
  • Stage I (0-24h): Nausea, vomiting.
  • Stage II (24-72h): RUQ pain, ↑LFTs, ↑INR.
  • Stage III (72-96h): Peak hepatotoxicity, encephalopathy, coagulopathy.
  • Stage IV (>4 days): Resolution or transplant/death.
• Diagnosis: Serum acetaminophen level (timed, ≥4 hours post-ingestion); plot on Rumack-Matthew nomogram.
  • 
• Management:
  • Supportive care. Activated charcoal (if <4h post-ingestion, 1g/kg).
  • Antidote: N-acetylcysteine (NAC) - replenishes glutathione & detoxifies NAPQI. IV/Oral routes.

  ⭐ N-acetylcysteine (NAC) is the specific antidote for acetaminophen poisoning and is most effective if administered within 8 hours of ingestion.

• Management Algorithm:

High‑Yield Points - ⚡ Biggest Takeaways

• Mechanism: Primarily central COX inhibition; minimal peripheral anti-inflammatory effect. Not a true NSAID.
• Key Uses: Preferred analgesic & antipyretic in children, pregnancy, peptic ulcer disease, and aspirin-induced asthma.
• Overdose Toxicity: Hepatotoxicity from toxic NAPQI metabolite due to glutathione depletion.
• Antidote: N-acetylcysteine (NAC); replenishes glutathione.
• Advantages: No significant GI distress, antiplatelet effects, or Reye's syndrome risk.
• Max Dose: Generally 4g/day (adults); lower with liver impairment or chronic alcohol use.