You should talk with your healthcare provider prior to taking this medication if you have:
- Liver disease, such as hepatitis, cirrhosis, or liver failure
- Kidney disease, such as kidney failure (renal failure)
- Been treated with a medicine known as thioguanine (Tabloid®)
- Been told you have thiopurine-S-methyltransferase (TPMT) deficiency or a defect in the TPMT gene
- Inflammatory bowel disease, such as Crohn's disease or ulcerative colitis
- Plans to receive a vaccination
- Any allergies, including allergies to foods, dyes, or preservatives.
Also, let your healthcare provider know if you are:
- Pregnant or thinking of becoming pregnant (see Purinethol and Pregnancy)
- Breastfeeding (see Purinethol and Breastfeeding).
Make sure to tell your healthcare provider about all other medications you are taking, including prescription and nonprescription medicines, vitamins, and herbal supplements.
(Click Precautions and Warnings With Mercaptopurine to learn more, including information on who should not take the drug.)
Mercaptopurine is similar in structure to purines, which are natural substances in the body that help make up DNA. Cells incorporate mercaptopurine into their structure, where the drug interferes with certain reactions that are important for DNA to replicate. Because DNA replication is necessary for cells to divide, mercaptopurine slows down the growth of cancer cells and causes the cancer cells to die.