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Immunosuppressant Pregnancy: D — teratogenic in high doses. BSG: can continue at standard IBD doses with consultant advice if benefit outweighs risk.

Azathioprine

Brand names: Imuran, Azathioprine

Adult dose

Dose: IBD maintenance: 2–2.5mg/kg/day. Autoimmune: 1–3mg/kg/day
Route: Oral
Frequency: Once daily
Max: 3mg/kg/day
IBD: start at 1–1.5mg/kg, titrate to 2–2.5mg/kg. Check TPMT enzyme activity before starting — TPMT deficiency: risk of severe myelosuppression. Onset of effect 8–12 weeks.

Dose adjustments

Renal

Reduce dose in moderate–severe renal impairment. Monitor FBC closely.

Hepatic

Use with caution in hepatic impairment — hepatotoxicity risk.

Clinical pearls

  • TPMT testing BEFORE starting: homozygous low/absent TPMT (~0.3%) → severe myelosuppression if started — do not use. Heterozygous (~10%) → start at 1mg/kg.
  • Monitor FBC weekly for first 8 weeks, then every 3 months. Stop if WCC falls below 3.5 × 10⁹/L.
  • Allopurinol interaction: azathioprine is metabolised by xanthine oxidase. Allopurinol blocks this → 4× increase in 6-thioguanine (myelosuppression) → may be intentional (thiopurine optimisation) but requires 75% dose reduction.
  • Takes 8–12 weeks to work in IBD — patients need bridging steroid therapy.

Contraindications

  • TPMT enzyme deficiency (homozygous — avoid entirely; heterozygous — start at 50% dose)
  • Active infection
  • Concomitant allopurinol (unless dose reduced by 75% — risk of severe myelosuppression)
  • Pregnancy (relative — discuss risk/benefit)

Side effects

  • Myelosuppression (leukopenia, thrombocytopenia) — dose-dependent
  • Hepatotoxicity (transaminase elevation)
  • Nausea and vomiting (start with meals, take at night)
  • Increased infection risk (opportunistic)
  • Pancreatitis (rare — idiosyncratic, usually in first weeks)
  • Lymphoma risk (long-term use)

Interactions

  • Allopurinol: severe myelosuppression — reduce azathioprine by 75% if combination unavoidable
  • Mesalazine / sulfasalazine: inhibit TPMT — increase azathioprine toxicity
  • ACE inhibitors: additive leukopenia risk
  • Warfarin: reduced anticoagulant effect

Monitoring

  • TPMT before starting
  • FBC weekly ×8 weeks then every 3 months
  • LFTs monthly ×3 months then every 3 months

Reference: BSG IBD Guidelines 2019; NICE NG130 IBD; NICE BNF 84. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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