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Thiopurine Immunomodulator Pregnancy: Caution — compatible in pregnancy for maintenance of IBD remission (benefit outweighs risk); discuss with specialist

Mercaptopurine (6-MP)

Brand names: Xaluprine

Adult dose

Dose: IBD: 1–1.5 mg/kg/day OD (after thiopurine methyltransferase [TPMT] testing)
Route: Oral
Frequency: Once daily
Max: 1.5 mg/kg/day (IBD); higher in haematology (per protocol)
Alternative to azathioprine for maintenance of remission in IBD. TPMT testing mandatory before starting — TPMT deficiency causes severe myelosuppression. 6-MP is active metabolite of azathioprine.

Paediatric dose

Dose: 1 mg/kg
Route: Oral
Frequency: Once daily
Max: 75 mg/day
Concentration: 20 mg/mL oral suspension mg/ml
Paediatric IBD: 1–1.5 mg/kg/day. Paediatric ALL maintenance: 50–75 mg/m² OD — specialist oncology protocol.

Dose adjustments

Renal

Reduce dose in renal impairment

Hepatic

Reduce dose in hepatic impairment — hepatotoxicity risk

Paediatric weight-based calculator

Paediatric IBD: 1–1.5 mg/kg/day. Paediatric ALL maintenance: 50–75 mg/m² OD — specialist oncology protocol.

Clinical pearls

  • TPMT testing before starting is mandatory — intermediate metabolisers need 50% dose reduction; homozygous deficient — avoid thiopurine entirely
  • Metabolite monitoring (6-TGN, 6-MMP levels) can optimise dosing and identify non-adherence or non-responders
  • Allopurinol interaction is critical — if needed together, reduce 6-MP to 25% of normal dose
  • Annual skin surveillance recommended (increased NMSC risk with long-term thiopurines)

Contraindications

  • TPMT deficiency (homozygous — high risk of fatal myelosuppression)
  • Concomitant allopurinol (unless dose reduced by 75%)
  • Active serious infection

Side effects

  • Myelosuppression (dose-dependent)
  • Hepatotoxicity
  • Nausea/vomiting
  • Pancreatitis
  • Increased lymphoma risk (long-term)
  • Opportunistic infections

Interactions

  • Allopurinol — inhibits xanthine oxidase, dramatically increases 6-MP levels (reduce 6-MP dose by 75% or avoid)
  • Warfarin — antagonises anticoagulant effect (increases warfarin dose requirement)
  • Live vaccines — contraindicated
  • Mesalazine/sulfasalazine — inhibit TPMT enzyme

Monitoring

  • FBC (weekly for 4 weeks, then monthly)
  • LFTs (monthly)
  • TPMT status before starting
  • Thiopurine metabolite levels (6-TGN/6-MMP)
  • Exacerbation frequency

Reference: BNFc; BNF; ECCO IBD Guidelines 2021; BNF for Children; MHRA Drug Safety Update on TPMT. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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