GASTROINTESTINAL SYSTEM DRUGS
EXAMPLES Ranitidine, cimetidine
MECHANISM OF ACTION
Competitive inhibitors of all histamine type 2 receptors. Inhibit histamine- and gastrin-stimulated gastric acid secretion by their action on parietal cells in the stomach.
INDICATIONS
Gastric and duodenal ulcers.
Gastro-oesophageal reflux.
Treatment and prophylaxis of NSAID-associated ulcers
CAUTIONS AND CONTRA-INDICATIONS
Hypersensitivity. If red flag features of malignancy, need to investigate prior to initiating treatment
SIDE-EFFECTS
GI disturbance, especially diarrhoea.
Gynaecomastia (cimetidine)
METABOLISM AND HALF-LIFE
These drugs are excreted largely unchanged in the urine. t½ is 2–3h.
MONITORING
No specific drug monitoring required.
DRUG INTERACTIONS
Cimetidine inhibits Cytochrome P450 activity in the liver and therefore potentiates the action of drugs such as warfarin, phenytoin, and theophylline
IMPORTANT POINTS
Ranitidine can be used prior to general anaesthesia in patients at high risk of aspiration particularly in obstetric practice (Mendelson's syndrome)
Laxatives
MECHANISM OF ACTION
Bulk laxatives (e.g. ispaghula husk) – polysaccharide polymers that are not broken down by digestion and thereby increase stool volume. This stimulates intestinal peristalsis (via the stretch reflex) as well as softening faeces.
Osmotic laxatives (e.g. lactulose, Movicol ) – these poorly absorbed solutes increase the volume of water in the bowel lumen by osmosis hence increasing transit.
Stimulant laxatives(e.g.senna,docusatesodium)–the primary effective direct stimulation of myenteric plexus resulting in smooth muscle contraction and increased peristalsis.
Faecal softeners(e.g.arachisoil)–these are surfactants that reduce surface tension and allow water to penetrate and soften faeces.
INDICATIONS
Constipation.
Hepatic encephalopathy (lactulose)
CAUTIONS AND CONTRA-INDICATIONS
Bowel obstruction.
Galactosaemia (lactulose only).
Acute inflammatory bowel disease.
Severe dehydration
SIDE-EFFECTS
Flatulence.
Diarrhoea.
Abdominal cramps.
Electrolyte disturbances
METABOLISM AND HALF-LIFE
Variable – most are broken down locally in the GI tract with minimal absorption.
MONITORING
No specific drug monitoring required.
DRUG INTERACTIONS
Lactulose may enhance warfarin effects in severe liver disease
IMPORTANT POINTS
Before prescribing a laxative ensure constipation is not secondary to an underlying pathology such as bowel cancer.
It requires at least 2–3 days for osmotic or bulking laxatives to take full effect.
Avoid laxatives if bowel obstruction is suspected due to the risk of perforation.
Lactulose reduces ammonia-producing organisms and is used in the treatment of hepatic encephalopathy.
Chronic laxative use can cause tolerance (stimulant laxatives)
Proton pump inhibitors (PPIs)
EXAMPLES Omeprazole, lansoprazole, esomeprazole, pantoprazole
MECHANISM OF ACTION
Inhibit Hþ/Kþ ATPase on the luminal surface of gastric parietal cells and thereby reduce gastric acid secretion.
INDICATIONS
Gastric/duodenal ulceration.
As part of triple therapy for eradication of Helicobacter pylori.
Gastro-oesophageal reflux disease.
Acid-related dyspepsia.
Hyper-secretory conditions, including Zollinger-Ellison syndrome.
Prevention and treatment of NSAID-associated ulcers
CAUTIONS AND CONTRA-INDICATIONS.
Hypersensitivity. If red flag features of malignancy need to investigate prior to initiating treatment
SIDE-EFFECTS
GI disturbance (e.g. abdominal pain, nausea, vomiting, diarrhoea) .
Increased risk of gastric infections due to hypochlorhydria
METABOLISM AND HALF-LIFE
Extensively metabolised by the liver and excreted by both renal and biliary routes. t½ varies from 40 min to 2h.
MONITORING
No specific drug monitoring required.
DRUG INTERACTIONS
PPIs are inducers of Cytochrome P450 with some variation in potency between the individual drugs and, therefore, may enhance effects of drugs metabolised by the liver (e.g. phenytoin, carbamazepine, warfarin)
IMPORTANT POINTS
PPIs reduce acid secretion by>90% and are therefore more effective at healing peptic ulcers than H2 receptor antagonists (which reduce acid secretion by 50–60%) .
IVPPIs can be used in the management of acute upper GI bleeds under specialist supervision
PPIs reduce acid secretion by>90% and are therefore more effective at healing peptic ulcers than H2 receptor antagonists (which reduce acid secretion by 50–60%) .
IVPPIs can be used in the management of acute upper GI bleeds under specialist supervision
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