I. Classification of Drugs Acting on the Digestive System

1. Drugs used in Peptic Ulcer Disease

   • Antacids

   • Antiflatulents

   • H2-receptor antagonists

   • Proton Pump Inhibitors (PPIs)

   • Cytoprotective agents

   • Prostaglandin analogues

   • Selective antimuscarinic agents

   • Chelate complexes (e.g., bismuth compounds)

2. Antispasmodics and GI Motility Stimulants

   • Anticholinergics

   • Musculotropic spasmolytics

   • Prokinetic agents (e.g., dopamine antagonists)

3. Antidiarrheal Agents

   • Adsorbents

   • Antimotility agents

   • Antisecretory agents

   • Microflora modulators

4. Laxatives and Purgatives

   • Bulk-forming

   • Osmotic

   • Stimulant

   • Lubricant (stool softeners)

   • Bowel cleansing agents

5. Nutritional Preparations

   • Parenteral fluids

   • Enteral nutrition

   • Vitamin and mineral supplements

6. Anal and Rectal Preparations

   • Suppositories (glycerin, bisacodyl)

   • Hemorrhoidal creams

   • Local anesthetics, vasoconstrictors

7. Emetics and Antiemetics

   • Centrally acting emetics (e.g., apomorphine)

   • Peripherally acting emetics

   • 5-HT3 antagonists, D2 antagonists, NK1 antagonists

8. Appetite Stimulants

   • Antihistamines

   • Cannabinoids

   • Corticosteroids

9. Drugs for Irritable Bowel Syndrome (IBS)

   • Antispasmodics

   • 5-HT3 antagonists (e.g., alosetron)

   • 5-HT4 agonists (e.g., tegaserod)

   • Secretagogues

   • Antibiotics (e.g., rifaximin)

🔶 II. Drugs Used in Peptic Ulcer Disease

1. Antacids

• Mechanism: Neutralize gastric acid → raise pH >4

• Types:

  • Systemic: Sodium bicarbonate

  • Non-systemic: Magnesium hydroxide, Aluminum hydroxide, Calcium carbonate

• Side Effects:

  • Mg → diarrhea; Al → constipation

  • Milk-alkali syndrome (Ca-containing)

2. Antiflatulents

• Example: Simethicone

• Mechanism: Defoaming action, reduces surface tension of gas bubbles

3. H2-Receptor Antagonists

• Examples: Ranitidine (withdrawn in many countries), Famotidine, Nizatidine

• Mechanism: Block H2 receptors on parietal cells → ↓ acid secretion

• Pharmacokinetics: Rapid absorption, renal elimination

• Clinical Use: PUD, GERD, Zollinger-Ellison syndrome

4. Proton Pump Inhibitors (PPIs)

• Examples: Omeprazole, Pantoprazole, Esomeprazole

• Mechanism: Irreversibly inhibit H⁺/K⁺-ATPase pump

• Use: PUD, H. pylori eradication, NSAID ulcers

• Long-term Risks: Hypomagnesemia, B12 deficiency, bone fractures

5. Cytoprotective Agents

• Examples:

  • Sucralfate: forms physical barrier over ulcer base

  • Colloidal bismuth subcitrate: antibacterial against H. pylori

• Use: Adjunct in ulcer therapy, radiation-induced ulcers

6. Prostaglandin Analogues

• Example: Misoprostol

• Mechanism: PGE1 analogue → ↑ mucus and bicarbonate, ↓ acid

• Use: NSAID-induced ulcers

• Caution: Abortifacient effect

7. Selective Antimuscarinics

• Example: Pirenzepine, Telenzepine

• Mechanism: M1 receptor antagonists → ↓ vagal acid stimulation

• Use: Not commonly used today due to superior PPIs

🔷 III. Antispasmodics and Motility Stimulants

1. Antispasmodics

• Anticholinergics: Hyoscine, Dicyclomine, Mebeverine

• Mechanism: Block muscarinic receptors → smooth muscle relaxation

• Use: IBS, colic

2. Prokinetics

• Examples: Metoclopramide, Domperidone, Cisapride, Prucalopride

• Mechanism: D2 antagonism, 5-HT4 agonism → ↑ ACh release

• Use: GERD, gastroparesis

• Adverse: Tardive dyskinesia (metoclopramide)

🔶 IV. Antidiarrheal Drugs

1. Adsorbents

• Examples: Kaolin, Pectin, Activated charcoal

• Mechanism: Adsorb toxins and bacteria

2. Antimotility Agents

• Examples: Loperamide, Diphenoxylate

• Mechanism: μ-opioid receptor agonists → ↓ peristalsis

3. Antisecretory Agents

• Example: Racecadotril (enkephalinase inhibitor)

4. Flora Modifiers

• Examples: Probiotics (Lactobacillus), Rifaximin

🔷 V. Laxatives and Purgatives

1. Bulk-forming

• Examples: Psyllium, Methylcellulose

• Mechanism: Absorb water → increased bulk

2. Osmotic Laxatives

• Examples: Lactulose, Magnesium sulfate, Polyethylene glycol (PEG)

• Mechanism: Draw water into bowel lumen

3. Stimulant Laxatives

• Examples: Bisacodyl, Senna

• Mechanism: Stimulate enteric nerves → ↑ motility

4. Lubricants/Stool Softeners

• Examples: Docusate sodium, Liquid paraffin

5. Bowel Cleansing Solutions

• Examples: PEG with electrolytes

• Use: Colonoscopy prep

🔶 VI. Nutrient Preparations

1. IV Fluids and Feeds

• Examples: Dextrose, Normal saline, Total parenteral nutrition (TPN)

• Uses: Critical care, GI dysfunction

2. Vitamins

• Fat-Soluble: A, D, E, K

• Water-Soluble: B-complex, C

• Clinical Use: Replacement therapy, deficiency states

🔷 VII. Anal and Rectal Preparations

1. Suppositories

• Laxative Suppositories: Glycerin, Bisacodyl

• Anti-inflammatory: Hydrocortisone for hemorrhoids

2. Topical Rectal Agents

• Use: Hemorrhoids, fissures

• Examples: Lidocaine, Zinc oxide, Witch hazel

🔶 VIII. Emetics and Antiemetics

1. Emetics

• Centrally Acting: Apomorphine

• Locally Acting: Ipecac syrup (rarely used)

• Use: Poisoning (limited today)

2. Antiemetics

• 5-HT3 antagonists: Ondansetron

• Dopamine antagonists: Metoclopramide, Domperidone

• NK1 antagonists: Aprepitant

• Antihistamines: Promethazine

• Anticholinergics: Scopolamine

• Steroids: Dexamethasone

🔷 IX. Appetite Stimulants

• Cyproheptadine: H1 blocker with antiserotonin effect

• Dronabinol: Cannabinoid

• Megestrol acetate: Progestin analogue

• Corticosteroids: For cancer cachexia

🔶 X. Drugs for Irritable Bowel Syndrome (IBS)

1. IBS with Diarrhea (IBS-D)

• Loperamide

• Alosetron (5-HT3 antagonist)

2. IBS with Constipation (IBS-C)

• Lubiprostone

• Linaclotide

• Tegaserod (5-HT4 agonist)

3. Broad-spectrum agents

• Rifaximin (for bloating and flatulence)

• Tricyclic antidepressants (for visceral pain modulation)

📚 References

1. Goodman & Gilman’s The Pharmacological Basis of Therapeutics

2. Katzung BG. Basic and Clinical Pharmacology

3. WHO Model List of Essential Medicines

4. Rang & Dale’s Pharmacology

5. Clinical Guidelines from American Gastroenterological Association (AGA)