Systemic Immunosuppressant — Eczema / Autoimmune Dermatoses Pregnancy: Should not be given to pregnant patients or those likely to become pregnant soon without careful risk-benefit assessment; substantial transplacental transmission occurs. Effective contraception in women during and for 1 month after; men for 3 months after. Avoid breast-feeding unless benefits outweigh risks.

Azathioprine

Brand names: Imuran, Azathioprine 25/50mg

Used in: Inflammatory Bowel Disease

Azathioprine is an immunosuppressant used in inflammatory and autoimmune conditions and to prevent transplant rejection.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Other (non-transplant) indications: starting 1–3 mg/kg/day

Route: Oral (IV only when oral route impractical)

Frequency: Once daily

Max: Up to 3 mg/kg/day (other/non-transplant indications)

Dermatological use falls under the SPC 'Other indications' category. Starting dosage 1–3 mg/kg bodyweight/day, adjusted within these limits to clinical response (which may not be evident for weeks or months) and haematological tolerance; reduce maintenance to lowest level compatible with maintaining response; maintenance range may be <1 to 3 mg/kg/day. Consider withdrawing if no improvement within three months. Transplant regimen (for context): up to 5 mg/kg/day day 1, maintenance 1–4 mg/kg/day. Xanthine oxidase inhibitor interaction: when allopurinol given concomitantly, give only 25% of the usual azathioprine dose. TPMT-deficient and NUDT15-variant patients generally require dose reduction. Consider reduced starting doses in renal or hepatic impairment. Take standardised with/without food; at least 1 hour before or 2 hours after milk/dairy. Note: azathioprine + long-wave ultraviolet light have a synergistic clastogenic effect (relevant to dermatology/phototherapy). Paediatric population: posology in children is the same as in adults.

Dose adjustments

Renal

Consider reducing starting doses in renal impairment; monitor for dose-related adverse effects. No specific dose recommendation given.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

US labelling (FDA)

Reference — US labelling, may differ from UK

DOSAGE AND ADMINISTRATION Renal Homotransplantation The dose of azathioprine tablets required to prevent rejection and minimize toxicity will vary with individual patients; this necessitates careful management. The initial dose is usually 3 to 5 mg/kg daily, beginning at the time of transplant. Azathioprine tablets are usually given as a single daily dose on the day of, and in a minority of cases 1 to 3 days before, transplantation. Dose reduction to maintenance levels of 1 to 3 mg/kg daily is usually possible. The dose of azathioprine tablets should not be increased to toxic levels because of threatened rejection. Discontinuation may be necessary for severe hematologic or other toxicity, …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2025-05-19. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Contraindications

Hypersensitivity to azathioprine or to any excipient
Hypersensitivity to 6-mercaptopurine should alert to probable hypersensitivity to azathioprine

Side effects

Bone marrow depression, leukopenia (very common)
Thrombocytopenia (common)
Nausea (common)
Alopecia (rare); photosensitivity, acute febrile neutrophilic dermatosis (Sweet's syndrome) (not known)
Life-threatening liver injury (rare); hypersensitivity, Stevens-Johnson syndrome and toxic epidermal necrolysis

Interactions

Xanthine oxidase inhibitors (allopurinol) — give only 25% of usual azathioprine dose
Ribavirin — co-administration not advised
Live organism vaccines — avoid until at least 3 months after end of treatment
TPMT inhibitors (olsalazine, mesalazine, sulfasalazine) — may exacerbate myelosuppression

Clinical monograph

How it works

It is metabolised to mercaptopurine derivatives that impair purine synthesis and lymphocyte proliferation.

Prescribing in practice

Check thiopurine methyltransferase (TPMT) activity before starting — low activity greatly increases the risk of severe bone-marrow suppression.
It interacts dangerously with allopurinol and febuxostat (xanthine oxidase inhibitors), needing a large dose reduction or avoidance.
There is an increased risk of infection and of certain malignancies, including skin cancer — advise sun protection.

Monitoring

Monitor FBC (and liver function) regularly, especially at initiation and after dose changes; check TPMT before starting.

Counselling the patient

Report sore throat, fever, or unexplained bruising or bleeding.
Tell any prescriber you take it before starting allopurinol or other medicines.
Use sun protection and attend your monitoring blood tests.

Evidence & guidelines

A steroid-sparing immunosuppressant across autoimmune disease and transplantation, with TPMT testing and the critical allopurinol interaction.

Reference: BAD Atopic Eczema Systemic Guidelines 2020; MHRA Drug Safety Update (TPMT); BSR/BAD shared guidelines; 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).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways