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Gastroprotective Pregnancy: Section 4.6: mesalazine is known to cross the placental barrier, but the limited data available on its use in pregnant women do not allow accurate assessment of possible adverse effects; it should therefore be used with caution during pregnancy and lactation when the potential benefit outweighs the possible hazards in the opinion of the physician. Animal studies do not indicate direct or indirect harmful effects with respect to pregnancy, embryonal/foetal development, parturition or postnatal development. Lactation: low concentrations of mesalazine and higher concentrations of its N-acetyl metabolite have been detected in human milk; the clinical significance is undetermined, so caution should be exercised in a nursing woman. Hypersensitivity reactions such as diarrhoea cannot be excluded — if the infant develops diarrhoea, breastfeeding should be discontinued.

Mesalazine (5-Aminosalicylic Acid)

Brand names: Asacol, Mezavant, Octasa, Pentasa

Used in: Inflammatory Bowel Disease

Mesalazine (5-aminosalicylic acid) is used to induce and maintain remission in ulcerative colitis and in some Crohn's disease.

Adult dose

Dose: Ulcerative colitis, ORAL route only (this SPC covers the 800 mg MR tablet and no other presentation). Mild acute exacerbation: three tablets (2.4 g) a day in divided doses. Moderate acute exacerbation: six tablets (4.8 g) a day in divided doses. Maintenance of remission of ulcerative colitis: up to three tablets (2.4 g) a day, once daily or in divided doses. The same SPC also covers maintenance of remission of Crohn's ileocolitis at up to three tablets (2.4 g) a day in divided doses. The rectal suppository and enema regimens this page also lists are NOT covered by this source and are not dosed here.
Route: Oral. Swallow whole with water; do not break, crush or chew the tablets before swallowing.
Frequency: Acute exacerbations: in divided doses. Maintenance of ulcerative colitis: once daily or in divided doses. Maintenance of Crohn's ileocolitis: in divided doses.
Max: 4.8 g daily (six 800 mg tablets) — the highest dose stated anywhere in this SPC, and it is the dose for a moderate acute exacerbation of ulcerative colitis, i.e. this page's own indication. No higher figure is given for any indication or duration.
VERBATIM section 4.2, in full: 'Swallow whole with water. Do not break, crush or chew the tablets before swallowing. ADULTS: Mild acute exacerbations of ulcerative colitis: Three tablets (2.4g) a day in divided doses. Moderate acute exacerbations of ulcerative colitis: Six tablets (4.8g) a day in divided doses. Maintenance of remission of ulcerative colitis: Up to three tablets (2.4g) a day once daily or in divided doses. Maintenance of remission of Crohn's ileocolitis: Up to three tablets (2.4g) a day in divided doses. ELDERLY: The normal adult dosage may be used unless renal function is impaired (see section 4.4). CHILDREN: Not recommended.' That is the complete section — there is no truncation and no further regimen. || WHAT THIS SPC DOES NOT SUPPORT. Its maintenance figure is 'up to' 2.4 g a day, with no stated lower bound. No induction figure below 2.4 g exists here either. || FORMULATION IS NOT INTERCHANGEABLE. These figures are tied to the 800 mg modified-release tablet. Section 4.4 explicitly contemplates patients 'on concurrent oral products which contain or release mesalazine' as a group needing closer renal monitoring, which is a reminder that mesalazine presentations differ and are dosed separately; do not transfer these numbers to Pentasa, Octasa, Mezavant or any rectal preparation. || PAEDIATRIC USE IS EXCLUDED BY THIS LABEL, not merely unstudied — section 4.2 states 'CHILDREN: Not recommended.' paedDose is therefore null by instruction of the source. || ELDERLY: the normal adult dosage may be used unless renal function is impaired.

Dose adjustments

Renal

No numeric dose reduction is given — this SPC limits or excludes use rather than adjusting the dose. Confirmed severe renal impairment (GFR less than 20 ml/min) is an outright contraindication (section 4.3). Section 4.4 verbatim: 'Asacol should be used with extreme caution in patients with confirmed mild to moderate renal impairment (see section 4.3).' Renal function must be measured before starting (serum creatinine) and monitored periodically thereafter (see monitoring). Mesalazine is excreted rapidly by the kidney, mainly as N-acetyl-5-aminosalicylic acid; in rats, large intravenous doses produce tubular and glomerular toxicity. Mesalazine-induced nephrotoxicity should be suspected in any patient developing renal dysfunction during treatment, and treatment should be discontinued if renal function deteriorates. If dehydration develops, normal electrolyte and fluid balance should be restored as soon as possible. Elderly patients may take the normal adult dosage unless renal function is impaired.

Hepatic

No numeric adjustment is given. Severe hepatic impairment is an outright contraindication (section 4.3). Section 4.4 has a 'Hepatic Failure' heading but the text is cut off at the source-fetch limit immediately after it, so no further hepatic guidance was retrieved from this bundle.

Dose independently re-sourced from UK Summary of Product Characteristics (SPC) via the eMC — clinician-reviewed. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

  • A history of sensitivity to salicylates or any of the ingredients; renal sensitivity to sulfasalazine
  • Confirmed severe renal impairment (GFR less than 20 ml/min)
  • Severe hepatic impairment
  • Gastric or duodenal ulcer, haemorrhagic tendency
  • PROVENANCE: section 4.3 of the Asacol 800mg MR Tablets SPC, complete in the bundle. Section 4.2 additionally states that this product is not recommended in children.

Side effects

  • Common: headache
  • Common: nausea, vomiting, diarrhoea, abdominal pain
  • In the phase III studies in moderate active ulcerative colitis, patients treated for 6 weeks with either 2.4 g/day or 4.8 g/day showed no difference in adverse event profiles between doses; the events reported in at least 2% of either group were headache (7.5% on 4.8 g/day vs 6.0%), abdominal pain (4.2% vs 5.1%), diarrhoea (3.8% vs 3.8%), nausea (3.8% vs 1.7%), respiratory infection (3.3% vs 1.7%), exacerbation of colitis (2.8% vs 2.6%), dyspepsia (2.8% vs 2.1%), vomiting (2.8% vs 0.9%), flatulence (2.3% vs 3.0%), rectal disorder (1.9% vs 2.6%), flu syndrome (1.4% vs 3.4%), rash (1.4% vs 2.1%), increased cough (0.5% vs 3.8%), sinusitis (0.5% vs 2.1%) and rhinitis (0.0% vs 3.0%)
  • Blood and lymphatic system: leucopenia, neutropenia, agranulocytosis, aplastic anaemia and thrombocytopenia
  • Immune system: hypersensitivity reactions
  • Nervous system: peripheral neuropathy, vertigo; idiopathic intracranial hypertension
  • Cardiac: myocarditis, pericarditis
  • Respiratory: bronchospasm, eosinophilic pneumonia, interstitial pneumonitis, pleurisy, allergic and fibrotic lung reactions
  • Gastrointestinal: pancreatitis; exacerbation of the symptoms of colitis; constipation
  • Hepatobiliary: abnormalities of hepatic function including hepatic failure, abnormal liver function test, hepatitis
  • Skin: alopecia, lupus erythematosus-like reactions, rash including urticaria, bullous skin reactions, erythema multiforme; DRESS, Stevens-Johnson syndrome and toxic epidermal necrolysis
  • Musculoskeletal: arthralgia/myalgia
  • Renal and urinary: interstitial nephritis, nephrotic syndrome, renal failure, nephrolithiasis
  • Reproductive: oligospermia (reversible)
  • General: drug fever; mesalazine-induced acute intolerance syndrome
  • FREQUENCY CAVEAT AND PROVENANCE: section 4.8 of the Asacol 800mg MR Tablets SPC. Only headache and the four gastrointestinal reactions above are unambiguously in the Common column; the remaining system-organ-class rows did not survive the fetch as aligned frequency columns, so no band is asserted for them. Section 4.8 is also truncated at the source-fetch limit, so this list is not guaranteed complete.

Monitoring

  • Serum creatinine before starting treatment, then renal function periodically during treatment — section 4.4 suggests every 3 months for the first year, then every 6 months for the next 4 years, and annually thereafter, based on individual patient history
  • Discontinue mesalazine if renal function deteriorates; suspect mesalazine-induced nephrotoxicity in any patient who develops renal dysfunction on treatment
  • Urinalysis, BUN and creatinine testing in patients on mesalazine 1000 mg, especially those on concurrent oral products which contain or release mesalazine and those with pre-existing renal disease
  • Standard haematological indices including the white cell count, monitored repeatedly, in patients taking azathioprine — especially at the beginning of combination therapy, whether or not mesalazine is prescribed. Mesalazine inhibits thiopurine methyl-transferase (TPMT) in vitro and may impair the metabolism of azathioprine, thioguanine and 6-mercaptopurine
  • Watch for the acute intolerance syndrome — cramping, acute abdominal pain and bloody diarrhoea, sometimes with fever, headache and a rash. Prompt withdrawal is required, and the patient's history of sulfasalazine intolerance should be re-evaluated
  • Ensure adequate fluid intake: cases of nephrolithiasis have been reported, including stones of 100% mesalazine content
  • Warn the patient that mesalazine may produce a red-brown urine discoloration after contact with sodium hypochlorite bleach in toilets — a harmless effect that is otherwise alarming
  • Use in the elderly should be cautious and subject to patients having normal renal function
  • This section 4.4 is truncated at the source-fetch limit (it stops just after the 'Hepatic Failure' heading), so further monitoring requirements beyond that point were not retrieved

Clinical monograph

How it works

It acts topically on the bowel mucosa with local anti-inflammatory effects; different formulations deliver it to different parts of the gut.

Prescribing in practice

  • Formulations release at different sites and are not interchangeable — prescribe by brand.
  • Rare but important renal toxicity (interstitial nephritis) and blood dyscrasias can occur.
  • It is generally far better tolerated than sulfasalazine (no sulfapyridine component).

Monitoring

Check renal function before starting and periodically; review the blood count if features of a dyscrasia occur.

Counselling the patient

  • Stay on the same brand.
  • Report unexplained bleeding, bruising, sore throat, or a marked change in urine output.
  • Take it regularly to maintain remission, even when you feel well.

Evidence & guidelines

First-line for inducing and maintaining remission in ulcerative colitis (NICE NG130).

Reference: BSG UC Guidelines 2019; NICE NG130 IBD; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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