What the label lists and what reports show
Mercaptopurine side effects
Mercaptopurine's FDA label: “The most common adverse reaction (> 20%) is myelosuppression including anemia, neutropenia, lymphopenia and thrombocytopenia.” FAERS holds 17,961 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is febrile neutropenia (1,923 reports).
- Generic listed
Check it at the source: the mercaptopurine FDA label on DailyMed (effective July 20, 2026, Nova Laboratories, Ltd); the FDA FAERS public dashboard.
What does the patient leaflet say about side effects?
From the patient leaflet: What are the possible side effects of MERCAPTOPURINE?MERCAPTOPURINE can cause serious side effects, including: Decreased blood cell counts are common with MERCAPTOPURINE, but can also be severe. MERCAPTOPURINE affects your bone marrow and can cause decreased white blood cells, red blood cells, and platelets. Decreased blood cell counts can make you more likely to develop infections, bleeding, or anemia. If you take certain medicines during treatment with MERCAPTOPURINE, it could make the effects on your bone marrow worse. Tell your healthcare provider if you develop any of the following symptoms during treatment with MERCAPTOPURINE: • fever • sore throat • cuts or wounds that are red, or swollen, or are draining • any bleeding • tiredness or weakness • shortness of breath Liver problems. […]
Source: FDA drug label, Nova Laboratories, Ltd, effective July 20, 2026 (SPL set 9c6fc66b-522f-499d-961a-cde6ca0ec2e5), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsThe most common adverse reaction (> 20%) is myelosuppression including anemia, neutropenia, lymphopenia and thrombocytopenia.
From the label: adverse reactions, continuedThe most common adverse reaction (> 20%) is myelosuppression including anemia, neutropenia, lymphopenia and thrombocytopenia. Adverse reactions occurring in 5% to 20% of patients include anorexia, nausea, vomiting, diarrhea, malaise and rash. ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Rare Disease Therapeutics, Inc., at 1-844-472-7389 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch. The following clinically significant adverse reactions are described elsewhere in the labeling: Myelosuppression [see Warnings and Precautions (5.1) ] Hepatotoxicity [see Warnings and Precautions (5.2) ] Immunosuppression [see Warnings and Precautions (5.3) ] Treatment Related Malignancies [see Warnings and Precautions (5.4) ] Macrophage Activation Syndrome [see Warnings and Precautions (5.5) ]
Source: FDA drug label, Nova Laboratories, Ltd, effective July 20, 2026 (SPL set 9c6fc66b-522f-499d-961a-cde6ca0ec2e5), via openFDA; the full label on DailyMed
What do FAERS reports show?
17,961 reports list mercaptopurine among the medicines taken, each a report of something that happened, not proof the medicine caused it; 1,979 are coded as a death and 7,995 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 22 reactions below, 7 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.
Reports received each year
The 25 most reported reactions
Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms
What next?
- Mercaptopurine: the full record
- label, generics, price, shortages, recalls
- Report a side effect to the FDA
- MedWatch
- Side effects hub
- the most reported medicines and reactions