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Generic Name

mercaptopurine (6-MP) (mer kap toh PYUR een)


Trade Name

Purinethol (50 mg. )
This drug is not available in a generic form.


What is mercaptopurine?

This drug was originally developed to treat certain forms of leukemia. But it also affects the immune system so that in some conditions where the immune system is overly active, such as Crohn’s disease or autoimmune hepatitis, the drug has found a role.


What is it used for?

In gastroenterology, the drug is used for Crohn’s disease, especially in treating draining fistulas. It may also have a role in autoimmune hepatitis and ulcerative colitis.


How do I take it?

Follow your physician’s instructions carefully. Mercaptopurine may be taken with food or milk to reduce stomach irritation. If nausea and vomiting develop, try eating small meals throughout the day consisting of dry foods (toast, crackers), soups or unsweetened juices. Drink two quarts of fluid a day to maintain a good urine flow. Tablets may be crushed and sprinkled on food. Store in a tightly closed container away from light. Keep all medications away from children. Never share your medications with anyone else.


What do I do for a missed dose?

If you miss a dose of this medicine, take it as soon as possible. If it is almost time for your next dose, skip the missed dose and return to your regular schedule. Do not double up on this medication.


Are there interactions with food or beverages?

There are no known interactions between this medication and foods or beverages.


Are there interactions with other drugs?

An interaction generally means that one drug may increase or decrease the effect of another drug. Also, the more medications a person takes, the more likely there will be a drug interaction.

Interactions with this drug may occur with the following:

  • blood thinners (Coumadin)
  • allopurinol (Zyloprim)
  • methotrexate (Rheumatrex)
  • sulfa drugs (Bactrim, Septra)


Is there a problem if I have another disorder or disease?

At times, a drug may have a different or enhanced effect when other diseases are present. At other times, the drug may worsen or effect another disease. The physician generally will test the red and white blood cells to be sure the proper amount of the drug is being taken. If anemia, a low white cell count, or bone marrow problems are already present, be sure to notify your physician. Also, alert your physician if you have a tendency to infections, have any liver condition or have had pancreatitis.


What about allergies?

People who have known allergies or asthma may be at an increased risk for a reaction from any new medication. The physician should always know a patient’s allergy history. Signs of an allergic reaction are skin rash, hives or itching. Of course, a person should not take mercaptopurine if there has been a previous reaction to this or other chemotherapy drugs.

There is a related drug called azathioprine (Imuran) that may have a cross reaction with mercaptopurine. You should discuss this with your physician.


What if I’m pregnant, considering pregnancy or breast-feeding?

Most females now know that, if possible, no drug, including alcohol, should be taken during pregnancy or lactation. The potential danger, of course, is an injury to the baby. However, some drugs are much safer than others in this regard. So, the FDA has a grading system for each drug which reflects what is known medically. It ranks drugs from A, where medical studies show no evidence for danger to the fetus or mother, to B, C, D and X, where the medical evidence indicates that the risk to the fetus outweighs any benefit to the mother. Mercaptopurine is ranked D. This drug can cause fetal harm or miscarriage so it should be used during pregnancy only if the benefit outweighs the risk to the fetus. Women of child-bearing potential should use effective contraception. It is not known if the drug is excreted in the breast milk. Because of its potential for serious harm to nursing infants, discontinuation of nursing or the drug must occur.


What are the effects on sexual function?

Mercaptopurine can suppress sperm production in males and can cause absence of menstruation in females.


Are there other precautions?

Inform your physician if you develop any signs of infection (fever, chills) or abnormal bleeding or bruising, or if you have been exposed to chicken pox or shingles. Avoid immunizations with live virus vaccines and people who have recently taken oral polio virus vaccine.


How long is it safe to take mercaptopurine?

This drug can be taken long-term. There was an initial concern about developing a tumor called lymphoma. In the early kidney transplant reports, there was an increased incidence of this tumor with this drug. However, in gastroenterology, the dose is only 1/3 to 1/2 that used in renal transplants. There have been no reports of increased malignancies above that seen in the general population. Long-term use requires periodic blood work and evaluation by a physician. It may take several months before the full effect of this drug occurs.


How about side effects?

Adverse reactions can occur with any drug, even over-the-counter medications. Some of these are mild such as a stomach upset, which may be avoided by taking the medication with food. Minor reactions may go away on their own but if they persist, contact the physician. For major reactions, the patient should contact the physician immediately.

For mercaptopurine, the following are the observed side effects:


  • loss of appetite
  • abdominal pain
  • sore mouth
  • nausea
  • vomiting
  • diarrhea
  • dark discoloration of skin
  • itching
  • rash


  • severe joint pain
  • severe upper abdominal pain (pancreatitis)
  • fever
  • unusual fatigue
  • weakness
  • sore throat
  • abnormal bleeding or bruising
  • yellow color to skin or eyes


A physician’s comment…

This drug is becoming increasingly important in the treatment of Crohn’s disease. It seems to be especially effective in closing draining fistulas around the anus. The drug should always be taken under a physician’s care.