Mechanism of Action

  • Active component is 5-aminosalicylic acide (5-ASA, or mesalazine)
    • Other forms include
      • Sulfasalazine- older; combination of 5-ASA and sulfapyridine (which acts as a carrier although causes side effects)
      • Balsalazide and Olsalazine
  • Mechanism of action not fully understood
    • Thought to ‘scavenge’ free oxygen radicals locally, and blocks production of prostaglandins and chemotaxis of leukotrienes (immune system modulation)


  • Mainly used in patients with Ulcerative colitis.  Also used, but to a lesser degree, in patients with Crohn’s disease (mainly ileocaecal disease).
    • Topically used for active proctitis
    • Can be used systemically if the patient has a poor response to topical treatment


  • MONITOR RENAL FUNCTION before, 3 months into and annually during treatment
    • Avoid in CKD 4/5 if possible (eGFR<20)
  • Avoid in severe liver disease
  • Avoid if allergic

Side effects

  • Blood disorders (dyscrasia)- quite rare but serious
    • Any patient should report any bleeding, bruising, purpura, sore throat, fever, malaise etc
    • Stop immediately if thought to be dyscrasia (perform FBC)
  • Lupus like syndrome
  • Other side effects include:
    • Diarrhoea, nausea, vomiting, abdominal pain, exacerbation of symptoms of colitis, headache, allergy
    • Rarer effects include: acute pancreatitis, hepatitis, myocarditis/pericarditis; Lung disorder (eosinophilia/fibrosing alveolitis); peripheral neuropathy, myalgia, arthralgia
    • Renal dysfunction (interstitial nephritis, nephrotic syndrome)
    • Skin reactions (lupus-like syndrome (malar rash); rarely Stevens-Johnson syndrome); alopecia

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