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This article is educational and does not replace your doctor's advice. Seek urgent care for any red-flag symptoms.
Drug class
Immunomodulator (thiopurine)
Examples
Azathioprine, 6-mercaptopurine (6-MP)
How it works
In inflammatory bowel disease (IBD), the immune system attacks the lining of the gut. These medicines gently turn down the activity of immune cells so there is less inflammation. They do not work quickly: it usually takes several weeks to a few months before the full benefit is felt, so you may need another medicine, such as a short course of steroids, in the meantime.
What it treats
- Keeping Crohn’s disease in remission (quiet phase)
- Keeping ulcerative colitis in remission, especially when steroids cannot be stopped
- Reducing the chance that antibodies form against some biologic medicines when used alongside them
- Other immune-related conditions, as decided by your doctor
How to take it
Take it exactly as your doctor prescribes, at the same time each day, usually with or after food to reduce nausea. The dose is chosen and adjusted by your doctor based on your weight, blood tests and response. Do not change or stop it on your own. If you miss a dose, follow the advice you were given, and never double up without asking. Keep every blood-test appointment.
Common side effects
- Nausea or an upset stomach (often eases when taken with food)
- Tiredness, headache or aching muscles
- Mild hair thinning in some people
- Mild changes in blood tests that your doctor will watch
- Getting infections more easily than before
Serious side effects — call your doctor
Fever, chills, sore throat or an infection that will not clear
The medicine can lower white blood cells that fight infection. Contact your doctor the same day; go to the emergency department if you feel very unwell.
Severe upper abdominal pain, with or without vomiting
May be inflammation of the pancreas (pancreatitis), usually early in treatment. Seek urgent medical care.
Yellow skin or eyes, dark urine, or severe itching
Can suggest liver irritation. Contact your doctor promptly.
Unusual bruising, bleeding, or extreme paleness and tiredness
May mean low platelets or red blood cells. Contact your doctor promptly; urgent care if bleeding does not stop.
New or changing skin lumps, spots or sores; swollen glands
Long-term immune-lowering medicines slightly raise the risk of some skin problems and rare blood cancers. Tell your doctor; do not panic, but do get checked.
Interactions
Tell your doctor and pharmacist about everything you take. Allopurinol (a gout medicine) can raise thiopurine levels dangerously unless the dose is changed by a doctor. Some other medicines, including certain blood-thinning and blood-pressure drugs and some antibiotics, can also interact. Avoid live vaccines while on treatment unless your doctor agrees, and ask which vaccines you should have, including yearly flu and other recommended vaccines. Sun protection is important.
Monitoring
Blood tests (blood count and liver tests) are done before starting, often every week or two at first, and then regularly (for example every few months) for as long as you take it. Some doctors check an enzyme called TPMT or NUDT15 before starting, which helps predict who is more sensitive. Regular skin checks and sun protection, and keeping up with recommended screening such as cervical screening, are advised.
When and how to stop
Never stop it suddenly on your own. Your doctor may pause or stop it if blood tests change, if you develop a serious side effect or infection, or if it is not helping. Stopping may allow the disease to flare, so any change should be planned with your doctor.
Pregnancy & breastfeeding
Many doctors continue thiopurines during pregnancy when needed, because an active IBD flare is also risky for mother and baby. Do not stop on your own. Talk with your gastroenterologist and obstetrician before planning a pregnancy, and tell them straight away if you become pregnant. Men planning to father a child should also discuss it with their doctor.
Questions to ask your doctor
Save this list or print it before your appointment — doctors appreciate prepared patients.
- How long before I can expect it to work, and what do I take in the meantime?
- How often will I need blood tests, and who will check the results?
- Which vaccines should I have, and which should I avoid?
- Do I need skin checks or other screening while on this medicine?
- How long will I need to stay on it?
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Status: Approved for publication by SGA; individual clinician review is ongoing · Published 24/04/2026
For general education only; it does not replace your doctor’s advice. Individual clinician review of this guide is ongoing. SGA clinician? Review this guide