The concomitant use of allopurinol with 6-mercaptopurine or azathioprine should be avoided because of a potentially dangerous interaction, the TGA has said in its latest safety advisory.

According to the advisory allopurinol reduces metabolism of 6-mercaptopurine and azathioprine, significantly increasing the risk of potentially fatal bone marrow toxicities and blood dyscrasias, including leukopenia, thrombocytopenia and pancytopenia.

“If these medicines must be given together, the dose of 6-mercaptopurine or azathioprine should be reduced to only one quarter of the normal dose, the statement advises.

Patients’ complete blood count should be closely monitored in accordance with the PI.