Azathioprine: immunosuppression, blood tests and sun care

The NHS describes azathioprine as an immunosuppressant that calms or controls the immune system. It helps treat inflammatory conditions such as rheumatoid arthritis, Crohn disease and ulcerative colitis, severe inflammation of the liver, skin or arteries, and some blood disorders. After a transplant it helps prevent the body rejecting the new organ. It is also sometimes used for lupus-related skin conditions or severe atopic eczema.

It slows production of new immune cells, which reduces inflammation. It is not a painkiller, but pain may ease as inflammation settles. It is prescribed by specialists as tablets, or as an injection in hospital.

The dose depends on body weight and why you need it. After a transplant the usual dose is 1mg to 2.5mg for each kilogram weighed, daily, usually for the rest of your life. For other conditions the usual starting dose is 1mg to 3mg each day for each kilogram, and your doctor may lower it as the condition improves.

Swallow tablets whole with water and do not chew them. You usually take them once or twice a day, with or without food, but not with milk or dairy products, which may lower azathioprine levels: take them at least 1 hour before or 2 hours after. If you forget one dose, take it unless the next is nearly due; check with a doctor or pharmacist if you miss two or more. Never double up.

Azathioprine can affect the liver, kidneys and bone marrow, so you have blood tests for liver function, kidney function and blood count before starting. From week 1 to week 8 these are at least once a week, particularly with a high dose or kidney or liver problems. From week 9 onwards they become less frequent, perhaps every few months, and monitoring continues for as long as you take it.

People who have ever been told they produce too little of the enzyme TPMT, or who have a rare inherited condition affecting the NUDT15 gene, should tell their doctor, as these affect suitability.

The most common effect, in more than 1 in 10 people when first starting or when the dose is increased, is mild nausea, which usually improves after a week or so; taking it after a meal or snack may help.

The NHS says to stop the medicine and speak to a doctor or call 111 if you feel tired all the time, dizzy or sick, or are vomiting or have diarrhoea; have a high temperature with shivering, cough or sore throat; have joint or muscle pain; notice changed urine colour or amount; feel confused, light-headed or weak; bleed or bruise more easily; notice lumps; or have severe stomach and back pain. Call 999 for anaphylaxis. Itching can signal a liver effect, particularly in pregnancy.

Taking it for a long time can increase the chance of certain cancers, including skin cancer, so avoid strong sunlight, use sunscreen (SPF 30 or more) and cover arms and legs. It can sometimes affect the liver, kidneys or bone marrow, which is why monitoring continues.

Because it weakens the immune system, infections are more likely. Avoid close contact with people who are unwell when possible, tell your doctor about a sore throat, high temperature or COVID-19, and have the vaccinations you are invited for, telling the vaccinator you take azathioprine. Live vaccines such as BCG are a concern, and children must not have the live children's flu vaccine.

You can take it in pregnancy, and it is particularly important to continue if you have an autoimmune condition or a transplant. After birth there is a very small risk it can affect how a baby fights infection, so tell clinicians. It passes into breast milk in tiny amounts and has been taken by many breastfeeding mothers without usual problems; babies might need extra blood tests at high doses. There is no evidence it reduces fertility.

Tell your doctor or pharmacist if you take allopurinol, ciclosporin or tacrolimus, warfarin, have chemotherapy, or will have surgery. Alcohol does not affect how it works, but both azathioprine and alcohol can affect the liver, so stick to 14 units a week or fewer, spread across 3 or more days.