DRUGS ACTING ON THE GASTRO INTESTINAL TRACT: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future pharmacologists and healthcare professionals!
The gastrointestinal tract is one of the most complex and vital systems in the human body, and drugs acting on it are among the most commonly prescribed medications worldwide. From anti-ulcer drugs that heal stomach ulcers to anti-emetics that control nausea, laxatives that relieve constipation, and anti-diarrhoeal drugs that manage diarrhoea, these drugs play a crucial role in managing GI disorders. As a pharmacology educator with years of experience teaching GI pharmacology, I have observed that students often find this topic challenging due to the various drug classes and their mechanisms. Let me tell you: Understanding GI drugs is essential for managing common digestive disorders that affect millions of patients worldwide.
In this comprehensive guide, I will walk you through the major classes of GI drugs—anti-ulcer drugs, anti-emetics, laxatives, and anti-diarrhoeal drugs. By the end of this article, you will have a thorough understanding of these essential pharmacological agents. Let us begin!
Dpharmguru’s exam insights:
GI drugs are frequently tested in pharmacology exams. Remember: Anti-ulcer drugs include H₂ antagonists (Ranitidine), Proton Pump Inhibitors (Omeprazole), Antacids, and Ulcer Protectives. Anti-emetics include Anticholinergics, H₁ Antihistaminics, Neuroleptics, Prokinetic drugs, and 5-HT₃ antagonists. Laxatives include Bulk-forming, Stool softeners, Stimulant, and Osmotic purgatives. Anti-diarrhoeal drugs include Anti-motility agents, Adsorbents, and Antispasmodics. These are classic exam questions!
1. ANTI-ULCER DRUGS
Anti-ulcer drugs are used in the treatment of peptic ulcers. Ulcers are sores or lesions that develop in the mucosal membrane and extend below the epithelium of the stomach (gastric ulcers) or duodenum (duodenal ulcers). Peptic ulcers develop due to an imbalance between the damaging effects of gastric acid and pepsin.
Classification of Anti-ulcer Drugs
- Anti-secretory Agents:
- H₂ Antagonists: Cimetidine, Ranitidine, Famotidine, Roxatidine, Loratidine
- Proton Pump Inhibitors: Omeprazole, Lansoprazole, Pantoprazole, Rabeprazole, Esomeprazole
- Anticholinergics: Pirenzepine, Propantheline, Oxyphenonium
- Prostaglandin Analogues: Misoprostol, Enprostil, Rioprostil
- Antigastrin: Octreotide
- Antacids:
- Systemic: Sodium bicarbonate, Sodium citrate
- Non-systemic: Magnesium hydroxide, Magnesium trisilicate, Aluminium hydroxide-gel, Magaldrate, Calcium carbonate
- Ulcer Protectives: Sucralfate, Colloidal Bismuth Subcitrate (CBS)
- Ulcer Healing Drug: Carbenoxolone sodium
- Anti-Helicobacter pylori Drugs: Amoxicillin, Clarithromycin, Metronidazole, Tinidazole, Tetracycline
H₂ Receptor Antagonists
Pharmacological Action: Block histamine action on parietal cells in the stomach, decreasing acid production.
Indications: Peptic Ulcer Disease (PUD), Dyspepsia, Zollinger-Ellison syndrome, GERD, Prevention of stress ulcer.
Adverse Effects: Headache, tiredness, dizziness, confusion, diarrhoea, constipation, rash, gynaecomastia in males, loss of libido, impotence.
Cimetidine
Dose: 800–1600 mg orally once a day at bedtime for up to 6 weeks.
Indications: Duodenal ulcers, active gastric ulcers, GERD, Zollinger-Ellison syndrome, heartburn, prevention of gastrointestinal bleeding.
Adverse Effects: Constipation, diarrhoea, fatigue, headache, insomnia, muscle pain, nausea, vomiting, confusion, hallucinations, breast enlargement, impotence, reduced WBC count.
Ranitidine
Dose: 150 mg orally 2 times a day or 300 mg orally once a day after the evening meal or at bedtime.
Indications: Stomach and intestinal ulcers, Zollinger-Ellison syndrome, GERD, heartburn.
Proton Pump Inhibitors (PPIs)
Pharmacological Actions: Irreversible inhibitors of the gastric parietal cell proton pump (H⁺/K⁺-ATPase). Reduce acid production by blocking the enzyme in the wall of the stomach that produces acid.
Indications: Peptic ulcers, GERD, Zollinger-Ellison syndrome, Stress ulcers, H. pylori eradication (with antibiotics).
Adverse Effects: Headache, diarrhoea, constipation, abdominal pain, nausea, rash.
Omeprazole
Dose: 20–40 mg 1–3 times a day taken before a meal.
Indications: GERD, excessive stomach acid production, erosive oesophagitis, heartburn, difficulty swallowing, persistent cough, H. pylori eradication.
Contraindications: Hypersensitivity to the drug or excipients.
Adverse Effects: Headache, diarrhoea, abdominal pain, nausea, dizziness, sleep deprivation.
Antacids
Pharmacological Actions: Increase gastric pH by neutralising acid; certain antacids form a protective covering over the gastric mucosa.
Indications: Symptomatic pain relief in gastric and duodenal ulcers, GERD (with alginic acid).
Adverse Effects:
- Sodium bicarbonate: Systemic alkalosis, gastric distension, rebound acidity, milk-alkali syndrome
- Magnesium salts: Diarrhoea, hypermagnesaemia (renal failure)
- Aluminium salts: Constipation, phosphate depletion, aluminium encephalopathy (renal failure)
Ulcer Protectives
Sucralfate: Complex salt of sucrose sulphate and aluminium hydroxide. Forms a viscous paste that adheres to the ulcer base selectively and protects against acid, pepsin, and bile.
Colloidal Bismuth Subcitrate (CBS): Forms a protective layer over the ulcer base; active against Helicobacter pylori.
Adverse Effects: Constipation, darkening of tongue, teeth, and stool.
2. ANTI-EMETICS
Anti-emetics are effective against nausea and vomiting. They are used to treat motion sickness and the side effects of opioid analgesics, general anaesthetics, and chemotherapy.
Classification of Anti-emetics
- Anticholinergics: Hyoscine, Dicyclomine
- H₁ Antihistaminics: Promethazine, Diphenhydramine, Dimenhydrinate, Cyclizine, Meclizine, Cinnarizine
- Neuroleptics: Chlorpromazine, Prochlorperazine, Haloperidol
- Prokinetic Drugs: Metoclopramide, Domperidone, Cisapride, Mosapride
- 5-HT₃ Antagonists: Ondansetron, Granisetron
- Adjuvant Anti-emetics: Dexamethasone, Benzodiazepines, Cannabinoids
Prokinetic Drugs
Metoclopramide
Pharmacological Actions: Promotes oesophageal and gastric peristalsis; increases tone of cardiac sphincter; accelerates gastric emptying; blocks CTZ (anti-emetic).
Dose: 10–15 mg up to 4 times a day, 30 minutes before meals and at bedtime.
Indications: Post-operative, drug-induced, disease-associated, radiation sickness vomiting; gastric stasis; dyspepsia; GERD.
Contraindications: Hypersensitivity, GI bleeding, perforation or haemorrhage, hypertension.
Domperidone
Pharmacological Actions: Dopaminergic antagonist; anti-emetic properties related to dopamine receptor blocking activity at CTZ and gastric level. Does not cross BBB significantly—rarely produces extrapyramidal side effects.
Dose: 10 mg 3–4 times per day.
Indications: Dyspepsia, heartburn, epigastric pain, nausea, vomiting.