M Toronto Myositis Centreat St. Michael's Hospital
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Azathioprine

Imuran · say “ay-za-THY-o-preen”
Immunosuppressant
iUsed in myositis based on specialist experience and evidence, not a formal Canadian myositis approval.
!
Never take with allopurinol.

Allopurinol (a gout medication) blocks the enzyme that clears azathioprine and can cause dangerous, potentially fatal bone-marrow suppression. Tell every doctor, dentist, and pharmacist that you take azathioprine.

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What it is

A steroid-sparing option, safe in pregnancy.

Azathioprine calms the overactive immune system that drives myositis and dermatomyositis. In these conditions the immune system mistakenly attacks your own muscle fibres and skin, causing weakness, inflammation, and rashes. Azathioprine reduces that activity to control the disease and prevent further muscle and organ damage. It has been used for over 50 years and is one of the few immune-suppressing medicines that can be continued safely in pregnancy.

How to take it
  • Available as 50 mg tablets. Usual dose is 50 to 200 mg daily, calculated by body weight (about 2 to 3 mg/kg/day).
  • Take once or twice daily as directed, with food to reduce stomach upset.
  • Swallow tablets whole; do not crush or chew.
  • Missed dose: take it when you remember; if the next dose is near, skip it and never double up.
  • Store at room temperature, away from children.
What to expect
  • It takes 6 to 12 weeks to reach full effect, and muscle strength may take longer to recover as fibres heal.
  • As it takes effect, your doctor will taper your steroid.
  • CK levels often improve before strength does. Do not stop because it seems slow.
Benefits

How it helps you.

Controls myositis inflammation

Suppresses the immune attack on muscle and skin, helping preserve muscle function.

Steroid-sparing

A main reason it is prescribed, letting your doctor taper prednisone and reduce steroid side effects.

Maintains remission

Once disease is controlled, it helps prevent flares and the return of weakness and rashes.

Protects muscle strength

Keeps inflammation in check so strength can recover with rehabilitation.

Improves skin disease

Helps control dermatomyositis rashes: heliotrope, Gottron’s papules, V-sign, and shawl sign.

Safe in pregnancy

One of the few immune-suppressing medicines usable in pregnancy and breastfeeding, important for women of childbearing age.

Risks & side effects

What you should know.

Your doctor prescribed this because the expected benefits outweigh the risks for your situation. Here is the honest, full picture.

Common side effects
Nausea and stomach upset
Most common. Taking with food and splitting the dose morning and evening often helps.
Diarrhea
Can occur early and usually settles as your body adjusts.
Mild hair thinning
Some slight thinning, usually mild and reversible with a dose change.
Higher infection risk
Suppresses the immune system. Stop for a fever or active infection and call your doctor.
Serious but uncommon
Liver effects
Detected by routine blood tests. Report yellowing of the skin or eyes, dark urine, or severe belly pain immediately.
Very low blood counts
Especially in people with low TPMT or NUDT15 enzyme activity. Regular blood monitoring is essential.
Cancer risk
Long-term use carries a small increased risk of skin cancers and lymphoma. Use sunscreen and attend skin checks.
Pancreatitis
Rarely, severe upper belly pain radiating to the back, usually early in treatment. Report immediately.
!
Never take allopurinol with azathioprine.

Allopurinol blocks the enzyme that breaks down azathioprine, which causes dangerous drug levels and potentially fatal bone-marrow suppression. Make sure every doctor, dentist, and pharmacist knows you take azathioprine.

Staying safe

Monitoring and precautions.

Regular monitoring
  • Blood tests initially every 1 to 2 weeks, then every 4 to 8 weeks once stable (CBC, liver enzymes, kidney function).
  • Blood counts are watched, since azathioprine can lower white cells, red cells, and platelets.
  • CK levels are tracked to monitor myositis activity alongside safety labs.
  • TPMT or NUDT15 genetic testing may be done before starting, since some people clear the drug slowly and need a lower dose.
Daily precautions
  • Never take allopurinol. Limit or avoid alcohol, since both affect the liver.
  • Avoid live vaccines. Flu and COVID vaccines are safe and recommended.
  • Use SPF 30+ sunscreen daily and attend annual skin checks, since long-term use raises skin-cancer risk.
  • Wash hands often and avoid close contact with people who are sick.
  • Tell every provider you take azathioprine before any prescription, procedure, or dental work.
When to call your doctor

Contact the clinic if you notice:

Fever or signs of infection
Any fever, chills, persistent cough, sore throat, or painful urination. Stop and call before the next dose.
New or worsening weakness
Trouble rising from a chair, climbing stairs, lifting your arms, or swallowing may signal a myositis flare.
Yellowing skin or eyes
Jaundice, dark urine, or severe belly pain may mean liver trouble. Stop and call immediately.
Unusual bleeding or bruising
Easy bruising, prolonged bleeding, or blood in urine or stool may signal low blood counts.
New skin rash or changes
A new rash could be medication-related or a dermatomyositis flare. Report new or changing moles too.
Prescribed allopurinol or new meds
Never take allopurinol with azathioprine. Check any new medication with your doctor.

Pregnancy & breastfeeding

Azathioprine can be continued safely during pregnancy and breastfeeding when needed for myositis control, which makes it an important option for women of childbearing age. Always tell your doctor if you are pregnant or planning.

!

This guide is for education, not medical advice. Myositis treatment is individual, and many of these medications are used based on specialist experience and research rather than a single approval for myositis. Never start, stop, or change a medication without your own doctor. Your doses and choices are decided with your care team at the Centre.

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