All medicines can cause unwanted side effects, although not everyone will get them. Estimates suggest side effects occur in around one in five people taking azathioprine and mercaptopurine, and can occur at any time during treatment.
Side effects from azathioprine and mercaptopurine may include the following:
Nausea (feeling sick), vomiting and loss of appetite. This reaction can be especially strong during the first few weeks of treatment. Taking your medication after eating or in two smaller doses each day instead of all at once as well as reducing dose may help reduce these side effects. Talk to your doctor or nurse specialist before you make any changes to your dose or how you take it
In some people, a flu-like illness with fever and general aches and pains a few days or weeks after starting treatment. Tell your doctor if you begin to feel unwell in this way. It is not usually serious but may mean the treatment has to be stopped
Suppression of normal bone marrow function, which can cause reductions of red blood cells (anaemia), white blood cells (leucopenia) and platelets (thrombocytopenia). This can lead to weakness, breathlessness and fatigue, a susceptibility to infections, and a tendency to bruise or bleed easily. Regular monitoring should pick up a reduced blood count but it is important that you contact your doctor if you develop an infection of any kind.
Liver inflammation. Your treatment monitoring will include regular liver function tests (LFTs) to check your liver is working properly. Tell your doctor if you notice a yellowing or discoloration of the skin which may be a sign of jaundice
Diarrhoea, which may be difficult to distinguish from symptoms of a flare-up
Pancreatitis (inflammation of the pancreas, a digestive gland in the abdomen), which can cause acute pain in the abdomen. Contact your doctor if you experience this type of symptom
Uncommon side effects include hypersensitivity reactions where people are allergic to the ingredients of the drugs, and cholestasis (where digestive juice cannot flow out from the liver). Rare side effects include sensitivity to sunlight, hair loss (which in many cases resolves spontaneously despite continuing treatment) and skin rashes.
A sore throat, unexplained bruising, bleeding, or mouth ulcers
Nausea, vomiting, abdominal discomfort, jaundice or dark urine
Central acute abdominal pain
Signs of an allergic reaction including hives (itchy wheals on the skin), difficulty breathing, swelling of the face, lips, tongue or throat
For people who have previously stopped treatment due to side effects, revisiting treatment may be possible, with one study suggesting around one in three people can restart treatment. Another study suggests that around seven out of 10 people intolerant of azathioprine can tolerate mercaptopurine.