What is azathioprine?

Azathioprine is an immunosuppressant that calms an overactive immune system. We use it for inflammatory muscle disease (myositis), lupus, mixed connective tissue disease (MCTD), vasculitis, and other autoimmune conditions. You may also know it by the brand name Imuran.

It belongs to a family of drugs called thiopurines. Your body converts azathioprine into building blocks that slow the production of the immune cells (lymphocytes) driving your inflammation. It is often used as a steroid-sparing medication — one that lets us lower your prednisone dose over time.

The TPMT test before your first dose

Important: We check a blood test called TPMT before you take your first azathioprine tablet. Do not start until we’ve reviewed the result.

TPMT is an enzyme that clears azathioprine from your body. A small number of people make very little of it. In those patients the drug builds up and can dangerously drop the blood counts. The TPMT test tells us whether azathioprine is safe for you and at what dose to start — some labs pair it with a related test called NUDT15.

The TPMT result guides your starting dose, but it does not replace ongoing blood work. Everyone on azathioprine still needs regular lab monitoring.

How to take it

Daily, as a tablet — this is a daily medication, not a weekly one.
We start low and increase the dose in steps toward your target (“goal”) dose, usually based on your weight.
Take it with food to reduce nausea. Splitting the dose into morning and evening can also help.
Swallow tablets whole; don’t crush or split them.

What to expect when starting

Azathioprine is slow-acting. Most patients begin to notice benefit over 8–12 weeks, sometimes longer. It works by gradually reducing inflammation, not by relieving symptoms quickly — so keep taking it even before you feel a change.

In the first few weeks some patients have mild nausea or stomach upset, which usually settles with food or a dose taken at night.

Call us in the first few weeks if you develop severe stomach or upper-abdominal pain with vomiting. Azathioprine can rarely inflame the pancreas (pancreatitis), and this needs prompt attention.

A small number of people have an early flu-like reaction — fever, muscle aches, and feeling unwell within the first weeks. Let us know if this happens; it usually means azathioprine isn’t the right fit.

Lab monitoring

You will need regular blood work on azathioprine:

Complete blood count (CBC) — checks your white cells, red cells, and platelets.
Comprehensive metabolic panel (CMP) — checks your liver and kidney function.

We check labs every 2 weeks while we raise your dose toward goal. Once you’ve reached your target dose and your counts have been stable for about three months, we space monitoring out to every 3 months for the long term.

This schedule is how we catch the two things azathioprine can do quietly — lower your blood counts or irritate your liver — before they cause symptoms.

Side effects to watch for

Fever, chills, sore throat, or other signs of infection — azathioprine can lower your white blood cells.
Unusual bruising or bleeding — a sign of low platelets.
Severe abdominal pain, nausea, or vomiting — could signal pancreatitis (see above).
Yellowing of the skin or eyes, or dark urine — possible liver irritation.
Unusual tiredness or shortness of breath — possible anemia.
A new or changing skin spot — see sun protection below.

Things to avoid

Allopurinol and febuxostat (gout medications). This is the most important interaction. These drugs block the enzyme that breaks down azathioprine, so the two together can cause the azathioprine level to climb to a dangerous point and crash your blood counts.

Important: Never start allopurinol or febuxostat while on azathioprine without telling us first. If you must take them together, we cut the azathioprine dose to about a quarter of usual and monitor closely.

Live vaccines — such as the live shingles vaccine (Zostavax) or the live nasal flu spray. Inactivated vaccines (flu shot, COVID, Shingrix) are safe and encouraged.
Excess sun. Azathioprine raises the long-term risk of skin cancer. Use sunscreen, wear a hat, and get any new or changing skin spot checked.
Alcohol — occasional, moderate use is generally fine, but heavy drinking adds stress to the liver we’re already monitoring.

Planning a pregnancy

Unlike many of our medications, azathioprine is one of the few immunosuppressants that can often be continued during pregnancy when the disease requires it. That decision is always made together — do not start, stop, or change it on your own if you are pregnant or planning to be. Tell us early so we can plan.

Frequently asked questions

Why do I need so many blood tests at the start? The first months are when azathioprine is most likely to drop your blood counts. Checking every two weeks lets us catch that early and adjust the dose. Once you’re stable, testing spaces out considerably.

Can I get vaccinated? Yes — inactivated vaccines (flu, COVID, Shingrix) are recommended. Avoid live vaccines, and talk to us before any new vaccination.

What if I miss a dose? Take it when you remember the same day. If it’s nearly time for the next dose, skip the missed one — don’t double up.

How long will I take it? Often for years, as long as it’s controlling your disease and your labs stay stable. We’ll revisit this as part of your ongoing care.

This handout is provided for educational purposes and does not replace individualized medical advice. Always follow the specific instructions given by your rheumatologist.

Azathioprine (Imuran): What You Need to Know