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Antacids and Mucosal Protectants

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Antacids Intro - Neutralizing Ninjas

  • Weak bases; chemically neutralize gastric HCl.
  • Result: ↑ gastric pH, ↓ pepsin activity (inactive > pH 4). Do NOT coat ulcers.
  • Mechanism: $HCl + Antacid \rightarrow Salt + H_2O (+ CO_2 \text{ if carbonate})$
  • Rapid onset, short duration. Symptomatic relief: dyspepsia, heartburn, GERD.
  • Classification:
    • Systemic: Absorbed. E.g., Sodium Bicarbonate ($NaHCO_3$).
      • ⚠️ Risks: Systemic alkalosis, fluid overload, milk-alkali syndrome.
    • Non-systemic: Poorly absorbed. E.g., Aluminium Hydroxide ($Al(OH)_3$), Magnesium Hydroxide ($Mg(OH)_2$), Calcium Carbonate ($CaCO_3$).
      • Longer action, local effects.

⭐ Systemic antacids like Sodium Bicarbonate can cause systemic alkalosis, while non-systemic antacids like Aluminium Hydroxide and Magnesium Hydroxide do not.

Antacid Agents - The Stomach Soothers

  • MOA: Weak bases; directly neutralize gastric HCl, raising pH. Symptomatic relief.
  • Onset: Rapid (minutes); Duration: Short (1-3 hrs). Best taken 1 & 3 hrs post-meals & at bedtime.
AgentANCOnsetKey Features & Side Effects
Magnesium HydroxideHighRapidDiarrhea (📌 MAG-nificent BM); hypermagnesemia (renal failure).
Aluminum HydroxideLow-ModSlowConstipation (📌 Alu-MINI-mum BM); hypophosphatemia, neurotoxicity (renal failure); binds phosphate.
Calcium CarbonateHighRapidConstipation, acid rebound; hypercalcemia.
Sodium BicarbonateModRapidSystemic absorption $\rightarrow$ alkalosis, fluid retention, $CO_2$ (belching). Avoid in HTN, CHF.
MagaldrateModRapidHydrolyzes to $Mg(OH)_2$ & $Al(OH)_3$; balanced bowel effects.
SimethiconeN/AN/AAntifoaming agent; often added to reduce gas.
-   ↓ Absorption (chelation/pH change): Tetracyclines, Fluoroquinolones, Ketoconazole, Itraconazole, Iron.
-   Administer antacids **1-2 hours** apart from other drugs.

⭐ Chronic use of Calcium Carbonate can lead to milk-alkali syndrome (hypercalcemia, alkalosis, renal impairment).

Mucosal Shields - Gut Guardians

Sucralfate coating ulcer in acidic environment

  • Sucralfate
    • Forms a viscous, sticky polymer in acidic environments (pH < 4), selectively binding to necrotic ulcer tissue.
    • Creates a physical barrier against acid, pepsin, and bile.
    • Stimulates prostaglandin synthesis and growth factors.
    • Minimal systemic absorption.
    • Adverse Effects: Constipation (most common), aluminum toxicity in renal failure.
    • Drug Interactions: Decreases absorption of phenytoin, digoxin, fluoroquinolones, ketoconazole. Administer other drugs 2 hours apart.

Sucralfate polymerizes and binds to necrotic ulcer tissue in an acidic environment (pH < 4), forming a protective barrier; its efficacy is reduced by co-administration with antacids, H2-blockers, or PPIs.

  • Colloidal Bismuth Compounds (e.g., Bismuth Subsalicylate)

    • Coats ulcers and erosions, creating a protective layer.
    • Antimicrobial action (including H. pylori).
    • Stimulates prostaglandin, mucus, and bicarbonate secretion.
    • Adverse Effects: Harmless blackening of stool and tongue. Salicylate toxicity (rare).
  • Misoprostol

    • Synthetic prostaglandin E1 (PGE1) analog.
    • MOA: ↓acid secretion, ↑mucus and bicarbonate secretion, ↑mucosal blood flow.
    • Primary Use: Prevention of NSAID-induced gastric ulcers.
    • Dose: e.g., 200 mcg QID.
    • Adverse Effects: Diarrhea, abdominal pain, uterine contractions.
    • ⚠️ Contraindicated in pregnancy (abortifacient).

High‑Yield Points - ⚡ Biggest Takeaways

  • Antacids (Al(OH)₃, Mg(OH)₂) neutralize gastric acid; Al(OH)₃ causes constipation, Mg(OH)₂ causes diarrhea.
  • Calcium carbonate risks hypercalcemia, rebound hyperacidity, and milk-alkali syndrome.
  • Sodium bicarbonate can cause systemic alkalosis; avoid in cardiac/renal disease.
  • Sucralfate forms a protective coat on ulcer base, requires acidic pH for activation, and chelates other drugs.
  • Bismuth subsalicylate offers antimicrobial action and coats ulcers; causes black stools and tongue.
  • Misoprostol (PGE₁ analog) protects against NSAID-induced ulcers; it's a potent abortifacient, contraindicated in pregnancy.

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Practice Questions: Antacids and Mucosal Protectants

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Misoprostol can be clinically used to treat gastric ulcers as at _____ doses it inhibits secretion of gastric acid

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Misoprostol can be clinically used to treat gastric ulcers as at _____ doses it inhibits secretion of gastric acid

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Antacids and Mucosal Protectants – NEET-PG Pharmacology Notes | Oncourse