H2 receptor Antagonists

Mechanism of Action

  • Blocks the H2 (histamine) receptor of parietal cells to prevent transport of H+ ions out of the cell into the stomach (acid formation)
  • H2-receptors-histamine

Indications

  • Used in GORD (particularly in patients intolerant of PPIs) and peptic ulcer disease
    • May also be used in functional dyspepsia and uninvestigated dyspepsia without worrying features
    • May also be indicated (high dose) for patients on dual NSAID/antiplatelet therapy (high risk of erosive gastritis) or those at risk due to a past history of erosive gastritis
  • May also be used for treatment of chronic urticaria

Contraindications

  • Relative
    • Do not give patients with warning signs of gastric cancer (e.g. weight loss, anaemia, malaena etc) as it can mask symptoms

Interactions

  • May alter the absorption and thus body levels of some drugs
  • It is also metabolised by P450 system, and slows metabolism of other drugs
    • Avoid in patients on warfarin, phenytoin or theophylline

Side effects

  • Diarrhoea, headache, dizziness are common
  • Uncommonly- rash (erythema multiforme and, rarer, toxic epidermal necrolysis)
  • Rarely, Hepatitis, cholestatic jaundice, bradycardia, psychiatric reactions (particularly in the elderly/very ill- may present as a delirium); blood count abnormalities (pancytopenia, leucopenia, thrombocytopenia); arthralgia/myalgia

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