Mercaptopurine (Purinethol, 6-MP) Side Effects

Ask your doctor about effects during and after taking this drug

Mercaptopurine is a drug known as a purine antagonist. It is used for the treatment conditions such as Crohn's disease and ulcerative colitis (two forms of inflammatory bowel disease [IBD]) and acute lymphocytic leukemia. It may be used for the treatment of other conditions as well.

6-mercaptopurine (6-MP) is typically taken once a day, and it could be given either in a tablet or an injection. 

Blue pills falling out of a pill bottle
Nehru Sulejmanovski / EyeEm/Gety Images

The Most Important Things to Know About 6-MP

  • 6-MP shouldn't be taken within two weeks of getting a live vaccine.
  • A lowered white blood cell count is a potential side effect.
  • A physician should do periodic blood tests to monitor liver function.
  • Women should use birth control to avoid pregnancy while taking 6-MP.
  • Breastfeeding is usually not recommended while taking 6-MP.

Important Information About 6-MP

The most important things to know about 6-MP is that it may cause certain side effects, such as hair loss, vomiting, diarrhea, and appetite loss. It should also not be used within two weeks of receiving a live vaccination. If you're unsure if a vaccination you received was live, check with your physician.

The hair loss can be very distressing, but it should reverse when the drug is stopped, and hair will begin to grow back normally. One study showed that 1% of people taking 6-MP to treat IBD experienced hair loss. More common was a lowered white blood cell count, which was found in about 11% of patients.

Of more urgent concern are side effects such as bleeding, bruising, or sores; dizziness or loss of consciousness; mouth pain; and joint pains. Some of these can also occur as signs of IBD, so it's important to keep your doctor in the loop on all the problems you're experiencing.

6-MP can also cause abnormal liver function tests, and in one study, this occurred in about 4% of patients taking it to treat IBD. For this reason, a doctor should do periodic blood tests to monitor liver function. In some cases, a high level of certain substances in the blood (aminotransferases) may mean that the dosage of the drug needs to be lowered, or that it needs to be discontinued.

It's recommended that women use birth control while taking 6-MP to avoid pregnancy. 6-MP is FDA Pregnancy Category D: there could be risks to the fetus, but the drug may be continued because stopping it may have more harmful consequences for the mother. If you become pregnant while taking 6-MP, tell your doctor. Breastfeeding a baby while taking 6-MP is not recommended.

Less Common Side Effects of 6-MP

Check with your doctor if any of the following side effects continue or are bothersome:

  • darkening of skin
  • diarrhea
  • headache
  • loss of appetite
  • skin rash and itching
  • weakness

Always Notify A Doctor Of These Side Effects

More common:

  • unusual tiredness or weakness
  • yellow eyes or skin

Less Common:

  • joint pain
  • loss of appetite
  • nausea and vomiting
  • swelling of feet or lower legs


  • Sores in mouth and on lips

Notify a Doctor Immediately About These Side Effects

Less common:

  • black, tarry stools
  • blood in urine or stools
  • cough or hoarseness
  • fever or chills
  • lower back or side pain
  • painful or difficult urination
  • pinpoint red spots on skin
  • unusual bleeding or bruising

Other Important Points

Because of the way medicines act on the body, there is a chance that they might cause other unwanted effects months or years after the medicine is used. These delayed effects may include certain types of cancer which should be discussed with a doctor.

Mercaptopurine may still produce side effects after stopping treatment. During this period of time, check with a doctor if any of the following side effects occur:

  • black, tarry stools
  • blood in urine or stools
  • cough or hoarseness
  • fever or chills
  • lower back or side pain
  • painful or difficult urination
  • pinpoint red spots on skin
  • unusual bleeding or bruising
  • yellow eyes or skin

Other side effects not listed above may also occur in some patients. If you notice any other effects, check with your doctor. This information is meant only as a guideline - always consult a physician or pharmacist for complete information about prescription medications.

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  1. Shaye OA, Yadegari M, Abreu MT, et al. Hepatotoxicity of 6-mercaptopurine (6-MP) and Azathioprine (AZA) in adult IBD patients. Am J Gastroenterol. 2007 Nov;102:2488-2494. doi:10.1111/j.1572-0241.2007.01515.x

  2. Mahadevan U. Fertility and pregnancy in the patient with inflammatory bowel diseaseGut. 2006;55(8):1198–1206. doi:10.1136/gut.2005.078097

  3. Glazier KD, Palance AL, Griffel LH, Das KM. The ten-year single-center experience with 6-mercaptopurine in the treatment of inflammatory bowel disease. J Clin Gastroenterol. 2005 Jan;39:21-26.

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