Azathioprine for Myositis

How does azathioprine work?

Azathioprine works by preventing overactive immune cells from making new DNA, which they need to grow and multiply.

• Slows down the production of new inflammatory immune cells
• Reduces the number of immune cells attacking muscles and other tissues
• Lowers inflammation over time
• Helps prevent long-term muscle damage

Uses

• Myositis
• Rheumatoid arthritis
• Systemic lupus erythematosus (SLE)
• Vasculitis and other autoimmune diseases
• Prevention of organ transplant rejection

How is azathioprine taken?

Azathioprine is usually taken once daily as tablets. Sometimes the daily dose may be divided into two doses to reduce stomach upset. Try to take it at approximately the same time each day.

Common side effects

Many people tolerate azathioprine well once their body adjusts. Common side effects include nausea, vomiting, loss of appetite, mild stomach discomfort, mild hair thinning, and fatigue.

Taking the medicine after meals or dividing the daily dose may help reduce stomach-related side effects.

Why might I need a TPMT or NUDT15 test?

Before starting azathioprine, your doctor may recommend a TPMT (Thiopurine Methyltransferase) or NUDT15 (Nudix Hydrolase 15) test. These tests look at how your body breaks down azathioprine.

Some people inherit genetic variations that make it difficult to process azathioprine safely. If this happens, the medicine can build up and increase the risk of very low white blood cell counts, infections, severe hair loss, and bone marrow suppression.

These tests are usually done once before treatment begins and do not need to be repeated.

Why are blood tests necessary?

Azathioprine can occasionally affect white blood cell counts, red blood cells and platelets, and liver function. Monitoring may include complete blood count (CBC) and liver function tests (LFTs).

Missed dose

Take the missed dose as soon as you remember. If it is almost time for the next dose, skip the missed dose and continue your usual schedule. Do not take a double dose.

During infection and sun protection

Contact your healthcare team if you develop high fever, a significant infection, hospital admission, or unexplained bruising or bleeding.

Wash hands regularly and avoid close contact with people who have contagious infections.

Azathioprine can make the skin more sensitive to sunlight. Use sunscreen regularly, wear protective clothing outdoors, and avoid excessive sun exposure.

Important drug interactions

Always tell your doctor about all medicines you are taking. Allopurinol and febuxostat (used to treat gout) can interact dangerously with azathioprine and may require major dose adjustments or avoidance.

Pregnancy, breastfeeding, and vaccines

Unlike MMF, azathioprine is often considered one of the safer immunosuppressive medicines during pregnancy when needed to control disease. Treatment decisions should always be individualised.

Azathioprine is generally considered compatible with breastfeeding in many situations, but this should always be discussed with your healthcare team.

Inactivated vaccines are generally safe. Flu and pneumococcal vaccination may be recommended. Live vaccines should usually be avoided while taking azathioprine.