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Systemic Immunosuppressant — Eczema / Autoimmune Dermatoses Pregnancy: Avoid — teratogenic in animal studies; used in transplant recipients under specialist guidance; BAD advises avoid in pregnancy

Azathioprine

Brand names: Imuran, Azathioprine 25/50mg

Adult dose

Dose: 1–3 mg/kg/day (based on TPMT enzyme activity)
Route: Oral
Frequency: Once or twice daily
Max: 3 mg/kg/day (2.5 mg/kg/day if TPMT intermediate metaboliser)
Steroid-sparing agent for severe atopic eczema, bullous pemphigoid, pemphigus vulgaris, dermatomyositis, and chronic actinic dermatitis. TPMT (thiopurine methyltransferase) genotyping/activity testing MANDATORY before starting — low/absent TPMT activity → severe myelosuppression at standard doses.

Paediatric dose

Dose: 1–3 mg/kg/day mg/kg
Route: Oral
Frequency: Once or twice daily
Max: 3 mg/kg/day
BNFc: atopic eczema in children — specialist use only; TPMT testing mandatory before initiation

Dose adjustments

Renal

Reduce dose in renal impairment — active metabolites accumulate

Hepatic

Avoid in severe hepatic impairment; use with caution and monitor LFTs

Paediatric weight-based calculator

BNFc: atopic eczema in children — specialist use only; TPMT testing mandatory before initiation

Clinical pearls

  • TPMT testing is MANDATORY before prescribing — absent activity (0.3% of population): do not prescribe; intermediate activity: use 50% of standard dose
  • Allopurinol interaction is potentially fatal — if patient is started on allopurinol while on azathioprine, reduce azathioprine dose by 75% immediately
  • Hypersensitivity syndrome (flu-like illness within 4 weeks): stop permanently — rechallenge risks severe recurrence
  • Slow onset of action: 8–12 weeks before full immunosuppressive effect — not suitable for acute flares
  • Lymphoma risk: EBV-associated lymphoma increased with prolonged azathioprine use — monitor for lymphadenopathy
  • Take with food to reduce nausea — divide into two doses if single daily dose causes GI upset

Contraindications

  • Absent TPMT activity (homozygous TPMT deficiency) — severe fatal myelosuppression
  • Pregnancy (teratogenic)
  • Hypersensitivity
  • Allopurinol co-administration without 75% dose reduction

Side effects

  • Myelosuppression (dose-dependent — especially low TPMT activity)
  • Nausea/vomiting (take with food)
  • Hepatotoxicity
  • Increased infection risk
  • Lymphoma (long-term — especially non-Hodgkin's)
  • Pancreatitis
  • Hypersensitivity syndrome (fever, rash, myalgia — within first month)

Interactions

  • Allopurinol — inhibits xanthine oxidase → dramatically increases azathioprine toxicity; reduce azathioprine dose by 75% if unavoidable
  • Warfarin — reduces anticoagulant effect (monitor INR)
  • ACE inhibitors — increased leucopenia risk
  • Co-trimoxazole — increased myelosuppression

Monitoring

  • FBC (weekly for first 4 weeks, then every 3 months)
  • LFTs (every 3 months)
  • TPMT activity (before starting)
  • Renal function
  • Lymph node assessment

Reference: BNFc; BNF 90; BNFc; BAD Atopic Eczema Systemic Guidelines 2020; MHRA Drug Safety Update (TPMT); BSR/BAD shared guidelines. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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