Azathioprine and Infection Risk: What Patients Need to Know
Nov, 18 2025
If you're taking azathioprine, you're likely managing a chronic condition like Crohn’s disease, ulcerative colitis, rheumatoid arthritis, or a transplanted organ. That’s already a lot to handle. But one of the most serious concerns patients quietly worry about is this: azathioprine makes you more likely to get infections-and not just a cold. It can open the door to serious, even life-threatening bugs.
How Azathioprine Weakens Your Defenses
Azathioprine doesn’t just calm down your immune system when it’s overreacting-it turns down the volume on your whole defense network. It stops white blood cells from multiplying and attacking what they think is a threat. That’s why it works for autoimmune diseases: your body isn’t attacking your own joints or gut. But the same mechanism means your body can’t fight off bacteria, viruses, or fungi the way it used to.
Studies from the American College of Gastroenterology show that patients on azathioprine have a 2 to 3 times higher risk of serious infections compared to those not on the drug. That includes pneumonia, tuberculosis, and even rare fungal infections like histoplasmosis. The risk isn’t the same for everyone-it depends on your dose, how long you’ve been taking it, your age, and whether you’re on other immunosuppressants too.
Common Infections You’re More Susceptible To
You don’t need to panic, but you do need to be aware. Here are the infections that show up more often in people taking azathioprine:
- Respiratory infections: Colds, flu, bronchitis, and pneumonia. If you’re coughing for more than a week or have a fever above 38°C, don’t wait it out.
- Herpes viruses: Shingles (herpes zoster) is a big one. About 1 in 20 people on azathioprine get it, compared to 1 in 100 in the general population.
- Urinary tract infections (UTIs): More frequent, sometimes harder to clear. If you feel burning when you pee or have cloudy urine, get tested.
- Skin infections: Cellulitis, abscesses, or fungal rashes that don’t go away with over-the-counter creams.
- Tuberculosis (TB): Even if you had TB years ago and were treated, azathioprine can reactivate it. That’s why screening before starting is required.
Some infections are rare but dangerous. For example, Listeria from deli meats or soft cheeses, or Cryptococcus from bird droppings. These aren’t everyday threats-but if your immune system is suppressed, they can spread fast.
What You Can Do to Lower Your Risk
You can’t stop azathioprine without talking to your doctor-but you can take smart steps to protect yourself.
- Get vaccinated: Make sure you’re up to date on flu, pneumonia (PCV20 and PPSV23), shingles (Shingrix), and COVID-19 vaccines. Avoid live vaccines like MMR or nasal flu spray-they can cause infection in people on immunosuppressants.
- Practice hygiene: Wash your hands often, especially before eating and after using the bathroom. Use hand sanitizer when soap isn’t available.
- Watch what you eat: Avoid raw or undercooked meat, unpasteurized dairy, raw shellfish, and soft cheeses like brie or camembert. Wash fruits and vegetables thoroughly.
- Avoid high-risk environments: Stay away from crowded places during flu season. Skip gardening if you have open cuts, and wear gloves when cleaning bird cages or compost piles.
- Know your body: If you feel unusually tired, have a fever, or notice a new rash or sore that won’t heal, call your doctor. Don’t wait for it to get worse.
When to Get Blood Tests and Why
Before you start azathioprine, your doctor should test your TPMT enzyme levels. This enzyme breaks down the drug. If you have low TPMT activity, even a standard dose can build up to toxic levels and crash your white blood cell count. About 1 in 300 people have this genetic variation.
After you start, you’ll need regular blood tests-usually every 2 to 4 weeks at first, then every 3 months. These check your white blood cell count, liver enzymes, and platelets. A sudden drop in white blood cells is your body’s first warning sign that infection risk is rising. If your count falls below 2.0 x 10⁹/L, your doctor may lower your dose or pause the drug.
Don’t skip these tests. Many patients think they feel fine, so they skip bloodwork. But infections can sneak in before you feel sick. Blood tests catch problems early.
What Happens If You Get Infected While on Azathioprine?
If you develop an infection, your doctor will likely pause azathioprine until you recover. That’s not always easy-some patients flare up their original condition when the drug is stopped. But the trade-off is necessary. In serious cases, you might need antibiotics, antivirals, or hospitalization.
For example, a 52-year-old woman in Dublin on azathioprine for rheumatoid arthritis developed a high fever and a red, swollen leg. She thought it was just a bruise. By the time she saw her doctor, she had sepsis from a skin infection. She spent 10 days in the hospital. Her azathioprine was stopped, and she needed IV antibiotics. She’s now back on a lower dose with closer monitoring.
Early action saves lives. If you’re on azathioprine, treat any fever or unusual symptom as a red flag.
Alternatives and When to Consider Them
If infections keep happening, or your white blood cell counts stay low, your doctor might switch you to another drug. Options include:
- Methotrexate: Lower infection risk than azathioprine, especially for arthritis.
- Biologics (like adalimumab or infliximab): More targeted, but still suppress immunity. They carry their own infection risks, but some are safer for people with recurrent UTIs or skin infections.
- Ustekinumab: Often used for Crohn’s or psoriasis. Appears to have a lower rate of serious infections in long-term studies.
There’s no perfect drug. But if you’re getting sick often, it’s worth discussing whether another option might give you better protection without losing control of your condition.
Living Well on Azathioprine
Many people stay on azathioprine for years without serious problems. The key isn’t fear-it’s awareness. You’re not powerless. You’re managing a balance: keeping your disease under control while protecting your body from invaders.
Keep a symptom journal. Note any fevers, rashes, or unusual fatigue. Bring it to your appointments. Ask your doctor: "Is my dose still right?" and "Should we check my TPMT again?"
Stay connected. Tell a family member or friend about your risk so they can help spot changes. Don’t isolate yourself-but be smart about exposure. Wear a mask in hospitals or during outbreaks. Avoid sharing utensils or toothbrushes.
Azathioprine isn’t a danger. It’s a tool. Used carefully, it gives people back their lives. But like any powerful tool, it demands respect-and attention.
Can azathioprine cause tuberculosis?
Yes. Azathioprine can reactivate latent TB, even if you were treated for it years ago. That’s why everyone starting the drug gets a TB skin test or blood test (IGRA) before beginning. If you’ve lived in or traveled to areas where TB is common-like parts of Asia, Africa, or Eastern Europe-this screening is especially important.
Is it safe to get the flu shot while on azathioprine?
Yes. The inactivated flu shot (the injection, not the nasal spray) is safe and recommended. It won’t cause the flu, and it reduces your risk of serious illness. Get it every year, ideally before flu season starts in autumn.
How long does it take for azathioprine to affect your immune system?
It takes about 6 to 12 weeks for azathioprine to reach its full effect on your immune system. But your white blood cell count can drop as early as 2 to 4 weeks after starting. That’s why blood tests start right away-before you even feel any changes.
Can I take antibiotics while on azathioprine?
Yes, but it depends on the antibiotic. Some, like trimethoprim-sulfamethoxazole, are often used to prevent infections in people on azathioprine. Others, like certain antivirals, may interact. Always tell your doctor you’re on azathioprine before starting any new medication, even over-the-counter ones.
Should I avoid pets while on azathioprine?
No, but be cautious. Avoid cleaning litter boxes or bird cages yourself. Wash your hands after petting animals, especially if they’re sick. Don’t let pets lick open wounds. The risk comes from animal waste or bites-not the pets themselves.
Final Thoughts: Stay Alert, Not Afraid
Azathioprine has helped millions live fuller lives. But it’s not a silent medication. It changes how your body works-and that means you have to be more alert. You’re not just taking a pill. You’re managing your health every day.
Know the signs. Get your blood tests. Ask questions. Don’t brush off a fever because you "feel okay otherwise." That’s when infections turn dangerous.
With the right precautions, you can stay healthy while staying on the drug that keeps your condition under control. The goal isn’t to avoid all risk-it’s to manage it wisely.