Azathioprine — what it does and when doctors use it
Azathioprine is a prescription immunosuppressant. Doctors prescribe it to prevent organ rejection after transplant and to control autoimmune conditions like Crohn's disease, ulcerative colitis, rheumatoid arthritis, and some types of lupus. It slows down the immune system so it stops attacking your body or a new organ.
It doesn’t work overnight. Expect slower, steady benefits — often several weeks to a few months for full effect. That’s why doctors usually combine azathioprine with faster-acting medicines at the start.
How it works and common dosing
Azathioprine is a purine analog: it interferes with DNA production in fast-growing immune cells, especially lymphocytes. Typical oral dosing is weight-based (often around 1–3 mg/kg/day), but your doctor will tailor the dose. If you’re also on allopurinol or febuxostat, the azathioprine dose usually must be cut dramatically because those drugs increase its active metabolites and raise the risk of bone marrow suppression.
Monitoring & safety — tests you should know about
Before starting azathioprine, most doctors test TPMT (thiopurine methyltransferase) activity or genotype. Low TPMT activity means a much higher risk of severe bone marrow suppression, so doses change or the drug is avoided. Baseline blood work should include a complete blood count (CBC) and liver tests. Expect frequent checks early on — for example, CBC every 1–2 weeks for the first month or two, then every 1–3 months once stable.
Watch for warning signs: fever, sore throat, easy bruising, unusual bleeding, extreme tiredness, or jaundice. Those can mean low blood counts or liver problems and need urgent lab checks or a call to your clinician.
Common side effects include nausea, vomiting, mild liver enzyme rises, and increased infection risk. Long-term use slightly raises the risk of certain cancers, especially skin cancers and lymphomas. Protect your skin from the sun and tell your doctor about any unusual lumps or persistent symptoms.
Live vaccines should be avoided while on azathioprine. Inactivated vaccines are usually safe but may be less effective. If you’re pregnant or planning pregnancy, talk with your specialist — azathioprine is sometimes continued in pregnancy when benefits outweigh risks, but that decision needs careful discussion.
Practical tips: take the pill with food to reduce nausea; don’t stop the drug suddenly without medical advice; carry an alert card saying you’re on an immunosuppressant; avoid close contact with people who have contagious infections. Keep all scheduled blood tests — they’re the best way to stay safe on this medicine.
If you have questions about dosing, interactions, or side effects, bring them up with your prescriber or pharmacist. Azathioprine can be very effective when used carefully, but it needs regular checks and respect for potential risks.
