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Mercaptopurine Side Effects

For the Consumer

Applies to mercaptopurine: oral suspension, oral tablet

In addition to its needed effects, some unwanted effects may be caused by mercaptopurine. In the event that any of these side effects do occur, they may require medical attention.

Major Side Effects

You should check with your doctor immediately if any of these side effects occur when taking mercaptopurine:

More common:
  • Abdominal or stomach pain or tenderness
  • black, tarry stools
  • blood in the urine or stools
  • clay colored stools
  • cough or hoarseness
  • dark urine
  • decreased appetite
  • fever or chills
  • headache
  • itching
  • loss of appetite
  • lower back or side pain
  • nausea and vomiting
  • painful or difficult urination
  • pinpoint red spots on the skin
  • skin rash
  • swelling of the feet or lower legs
  • unusual bleeding or bruising
  • unusual tiredness or weakness
  • yellow eyes or skin
Less common:
  • Bleeding gums
  • chest pain
  • joint pain
  • pale skin
  • shortness of breath
  • sore throat
  • sores, ulcers, or white spots on the lips or in the mouth
  • swollen glands
  • troubled breathing with exertion
Incidence not known:
  • Abdominal or stomach cramping or burning
  • constipation
  • diarrhea
  • heartburn
  • indigestion
  • joint pain, stiffness, or swelling
  • lower back or side pain
  • vomiting of blood or material that looks like coffee grounds

Minor Side Effects

Some of the side effects that can occur with mercaptopurine may not need medical attention. As your body adjusts to the medicine during treatment these side effects may go away. Your health care professional may also be able to tell you about ways to reduce or prevent some of these side effects. If any of the following side effects continue, are bothersome or if you have any questions about them, check with your health care professional:

Less common:
  • Darkening of the skin
  • headache
  • weakness
Incidence not known:
  • Hair loss or thinning of the hair
  • low sperm count

For Healthcare Professionals

Applies to mercaptopurine: oral suspension, oral tablet


Hematologic effects including myelosuppression have been reported. Myelosuppression is the most frequent adverse reaction reported with the use of mercaptopurine. Anemia, leukopenia, and thrombocytopenia have also been reported frequently.[Ref]


Hyperuricemia may occur as a consequence of rapid cell lysis. Adverse effects may be minimized by increasing hydration, urine alkalinization, and the prophylactic administration of a xanthine oxidase inhibitor (such as allopurinol).[Ref]

Renal effects including hyperuricemia have been reported.[Ref]


Gastrointestinal effects including intestinal ulceration have been reported. Nausea, vomiting, and anorexia have been infrequently reported during initial administration. Mild diarrhea and sprue-like symptoms have been reported occasionally. Oral lesions have been reported rarely. An increased risk of pancreatitis may be associated with the investigational use of mercaptopurine in inflammatory bowel disease.[Ref]

The mild diarrhea and sprue-like symptoms are not necessarily related to the mercaptopurine.

Oral lesions resemble thrush rather than antifolic ulcerations.[Ref]


Hepatic effects including a small number of deaths which may have been attributed to hepatic necrosis have been reported.[Ref]


Dermatologic effects have included skin rashes, alopecia, and hyperpigmentation.[Ref]


Genitourinary side effects including oligospermia have been reported.[Ref]


Other effects including drug fever have been reported very rarely.[Ref]

Before the drug fever is assumed to be caused by mercaptopurine, the clinician should exclude more common causes of pyrexia such as sepsis in patients with acute leukemia.[Ref]


Oncologic side effects have included hepatosplenic T-cell lymphoma.


1. "Product Information. Purinethol (mercaptopurine)." Glaxo Wellcome, Research Triangle Pk, NC.

2. Cappell MS, Das KM "Rapid development of pancreatitis following reuse of 6- mercaptopurine." J Clin Gastroenterol 11 (1989): 679-81

3. Laidlaw ST, Reilly JT, Suvarna SK "Fatal hepatotoxicity associated with 6-mercaptopurine therapy." Postgrad Med J 71 (1995): 639

4. Rehr EL, Swanson KA, Kern JA "Mercaptopurine-induced fever in a patient with Crohn's disease." Ann Pharmacother 26 (1992): 907-9

Not all side effects for mercaptopurine may be reported. You should always consult a doctor or healthcare professional for medical advice. Side effects can be reported to the FDA here.

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