6-mercaptopurine is a toxic drug used as a chemotherapy agent to treat acute forms of leukemia (lymphocytic and granulocytic) by inhibiting DNA synthesis. It also suppresses the immune system and is used to treat Crohn's disease.

This chemical is a purine analog which can cause genetic damage when it is mistakenly incorporated into a DNA strand during synthesis. It can also cause reduced blood cell counts and inflammation of the liver and pancreas.

Part of the information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and believe I wrote the entry this writeup is partly or wholly based upon.