Azathioprine and allopurinol: a blood-count interaction needing dose changes

The NHS describes azathioprine as an immunosuppressant that helps to calm or control the immune system. It treats inflammatory conditions such as rheumatoid arthritis, Crohn's disease and ulcerative colitis, severe inflammation of the liver, skin or arteries, and some blood disorders, and in people who have had a transplant it can prevent the body rejecting the new organ.

Allopurinol is described by the NHS as a medicine mainly used to treat gout. It helps reduce the amount of uric acid in the body and can be used to prevent gout attacks, some types of kidney stones and problems caused by uric acid build-up during some cancer treatment.

A person can reasonably need both, for example someone with an inflammatory condition who also has gout, which is why the combination is a recognised one with its own dose guidance.

The BNF interaction list for azathioprine contains an allopurinol entry stating that allopurinol potentially increases the risk of haematological toxicity when given with azathioprine. The manufacturer advice is to adjust the azathioprine dose, the severity is severe and the evidence type is study. The allopurinol list repeats it from the other side.

Haematological toxicity means harm to the blood cells, which can include lower white cells, red cells or platelets. Because azathioprine already affects the bone marrow, the NHS points to regular blood tests during treatment, and the combination calls for those checks to be particularly attentive.

The wording adjust the dose is different from avoid. The BNF is describing a combination that can be used, with the azathioprine dose changed by the prescriber, not something to try on your own.

The NHS azathioprine page says to stop taking the medicine and speak to a doctor or call 111 now if you feel tired all the time, dizzy or sick, have a high temperature with shivering or chills, a cough or a sore throat, have joint or muscle pain, notice bleeding or bruising more easily than usual, have changes in your pee, feel confused, light-headed or weak, notice lumps anywhere on your body, or have severe stomach and back pain.

Many of these could have other causes, but in someone taking azathioprine and allopurinol a high temperature, sore throat or unusual bruising deserves a same-day call. The NHS says it is important to ask for urgent advice if you get an infection while taking azathioprine.

• High temperature, shivering or sore throat.

• Unusual bruising or bleeding.

• Feeling very tired, weak or dizzy.

• Changes in the colour or amount of urine.

If allopurinol is being started in someone taking azathioprine, the prescriber should be told and the dose plan agreed before the first allopurinol tablet. If you are already taking both and are unsure whether the azathioprine dose was reviewed, ask the specialist team or pharmacist. Do not change the azathioprine dose yourself.

Keep to your blood test schedule, bring your monitoring booklet, and tell staff you take azathioprine before any vaccination, since the NHS page asks you to mention it, especially for live vaccines such as BCG.

In the NHS azathioprine is generally started and monitored by a specialist team with the GP sharing care, and pharmacists can check new prescriptions. Private specialists use the same BNF, and the key safeguard is the same: a current list of all medicines in every setting.