Azathioprine damps down an overactive immune system. It is used in inflammatory bowel disease, rheumatoid arthritis, autoimmune hepatitis and after transplants. It takes months to work fully, and it needs consistent blood monitoring for as long as you take it.
You should have a TPMT enzyme test before starting. People with low or absent TPMT activity break azathioprine down poorly and are at much higher risk of severe bone marrow suppression — the result changes the dose or the choice of medicine.
It can take eight to twelve weeks, sometimes longer, before you feel the benefit. Do not conclude it is not working before then.
Regular blood tests — weekly or fortnightly at first, then usually every three months — check your blood count and liver function. Treatment is not continued without them.
Take it with or just after food to reduce nausea, at the same time each day.
Azathioprine increases the risk of skin cancer. Use high-factor sun cream all year, cover up, avoid sunbeds, and check your skin regularly — report any new or changing mole or lesion.
Do not take allopurinol alongside azathioprine without specialist supervision — the combination requires a substantial dose reduction and can otherwise be dangerous.
Discuss pregnancy plans with your specialist rather than stopping on your own; azathioprine is sometimes continued in pregnancy where the alternative is a disease flare.
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