The haematological effects of
azathioprine are markedly increased by the concurrent use of
allopurinol.
Azathioprine is rapidly and extensively metabolised to mercapto- purine. Mercaptopurine is therefore expected to share the interactions of
azathioprine.
The dosage of
azathioprine or mercaptopurine should be reduced by two-thirds to three-quarters to minimise the risk of
toxicity. Despite taking these precautions
toxicity may still be seen and very close haematological monitoring is advisable if concurrent use is necessary.