Here are the potential side effects of Imuran (azathioprine) that I found with a simple web search (emphasis mine):
Are there any side effects?
Azathioprine can be very effective in providing remission from the symptoms caused by inflammatory bowel disease, and in allowing smaller doses of steroids to be used effectively whilst reducing the risk of relapse and steroid induced side effects. However it is possible that some side effects may occur during treatment. Symptoms, such as diarrhoea, nausea, mild aches and pains may occur initially but should settle within 2 weeks. Others, which are potentially more serious, may occur. These may include some of the following:
• Feeling generally unwell
• Dizziness
• Vomiting
• Fever
• Rigors
• Muscle pains
• Joint pains
• Abnormal liver function
• Jaundice
• Disturbances in heart rhythm
• Low blood pressure
• Kidney problems
• Skin rash (didn't you ask about this yesterday?)
These may be due to a hypersensitivity reaction and you may need to stop the treatment. You must contact your doctor or IBD nurse immediately for further advice.
Other reactions may occur such as:
• Reduced bone marrow activity i.e. low white cells [neutrophils are particularly important in fighting infection], low platelets [important in helping the blood to clot], low haemoglobin [iron in the blood].
• Hair loss