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Purinethol Side Effects

Generic Name: mercaptopurine

Note: This page contains side effects data for the generic drug mercaptopurine. It is possible that some of the dosage forms included below may not apply to the brand name Purinethol.

For the Consumer

Applies to mercaptopurine: oral suspension, oral tablet

As well as its needed effects, mercaptopurine (the active ingredient contained in Purinethol) may cause unwanted side effects that require medical attention.

Major Side Effects

If any of the following side effects occur while taking mercaptopurine, check with your doctor immediately:

More common:
  • Black, tarry stools
  • clay colored stools
  • cough or hoarseness
  • dark urine
  • decreased appetite
  • fever or chills
  • headache
  • loss of appetite
  • lower back or side pain
  • nausea, vomiting
  • painful or difficult urination
  • pinpoint red spots on the skin
  • rash, itchy skin
  • stomach pain or tenderness
  • 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
  • sore throat
  • sores, ulcers, or white spots on the lips or in the mouth
  • swollen glands
  • trouble with breathing upon exertion
Incidence not known:
  • Constipation
  • diarrhea
  • heartburn
  • indigestion
  • joint pain, stiffness, or swelling
  • stomach cramping or burning
  • vomiting of blood or material that looks like coffee grounds

Minor Side Effects

Some mercaptopurine side effects may not need any medical attention. As your body gets used to the medicine these side effects may disappear. Your health care professional may be able to help you prevent or reduce these side effects, but do check with them if any of the following side effects continue, or if you are concerned about them:

Less common:
  • Darkening of the skin
  • headache
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 (the active ingredient contained in Purinethol) 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 (the active ingredient contained in Purinethol) 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

It is possible that some side effects of Purinethol may not have been reported. These can be reported to the FDA here. Always consult a healthcare professional for medical advice.

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