Drug · FAERS side-effect profile with label screen
Mercaptopurine side effects: what 17,243 FDA adverse event reports show
Drug class
Nucleoside metabolic inhibitor
FDA label, Indications wording
Who reports
17,243 reports · 86% serious · 1,957 with death outcome · 48% female
openFDA FAERS counts, all years, deduplication not applied
Used for
Mercaptopurine is a nucleoside metabolic inhibitor indicated for the treatment of patients with acute lymphoblastic leukemia (ALL) as part of a combination chemotherapy maintenance regimen. ( 1.1 ) 1.1 Acute Lymphoblastic Leukemia Mercaptopurine is indicated for the treatment of patients with acute lymphoblastic leukemia (ALL) as part of a combination chemotherapy maintenance regimen.
FDA label, Indications and Usage
What it is
Mercaptopurine is a nucleoside metabolic inhibitor.
FDA label, Description section
Boxed warning
None in the current label
How to read this page. FAERS is a spontaneous-reporting system: counts are not incidence rates and a signal is not proof of causation.
Each reaction below is run through an automated screen — is the term in the FDA label (known), in the indications (the disease being treated), an administrative/coding term, or none of these (unlabeled)?
That screen is mechanical text matching, not a causality assessment. The 30 hand-written Bradford Hill assessments are on the causality assessment page. Not medical advice.
Causality screen of the 40 strongest signals
Labeled adverse reaction 1Labeled (related wording) 1Likely indication confounding 2Lab value / disease marker 3Not in label — unassessed 33
What the badges mean
- Labeled adverse reaction The exact term appears in the label's warnings or adverse-reactions sections: a known, expected effect. The FAERS signal confirms the label rather than discovering anything.
- Labeled (related wording) The label describes this effect under different wording (e.g. 'neutropenia' for 'neutrophil count decreased'). Treated as known.
- Likely indication confounding The term appears in the label's indications: reports probably describe the disease being treated (or its progression), not a drug effect. Classic confounding by indication.
- Lab value / disease marker A laboratory result, investigation or disease-activity marker (antibody positive, CRP abnormal, swollen joint count) not described in the label. These usually mirror the monitored disease or testing protocol rather than a drug effect; not screened further.
- Not in label — unassessed Not found in the label text, not an artifact pattern. This is a genuine candidate for a formal causality assessment (Bradford Hill / WHO-UMC); it has NOT had one here.
Strongest disproportionality signals
Flagged terms (PRR ≥ 2, χ² ≥ 4, n ≥ 10), ranked by PRR; infinite PRRs (no comparator reports) excluded. The EBGM column is a second, shrinkage-based estimate (MGPS) with its 5th percentile EB05 and the BCPNN IC025; a pair flagged by PRR alone is marked PRR-only and should be read as fragile. The strata column recomputes the PRR on serious reports only and flags pairs whose reports are unusually skewed by sex or come mostly from consumers rather than clinicians. 1,107 further terms are in the interactive explorer. Reaction links open the term's cross-drug page.
Show:
| Reaction | Reports | PRR 95% CI · ROR · χ²/p | EBGM EB05 · IC025 | Label screen | Serious-only & strata | Trial cross-check |
|---|---|---|---|---|---|---|
| Human bocavirus infection | 130 | 2,687.9 1,973.1–3,661.5ROR 2,708.3 · χ² 106,893 / p <1e-300 | 739.0 EB05 638.4 · IC025 +7.3 3/3 methods agree | Not in label — unassessed | serious n=130 · ♀ 100% (drug 48%) | no trial arm names this drug |
| Hcov-hku1 infection | 11 | 1,319.1 560.3–3,105.6ROR 1,320.0 · χ² 6,285 / p <1e-300 | 300.4 EB05 179.0 · IC025 +3.4 3/3 methods agree | Not in label — unassessed | serious n=11 | no trial arm names this drug |
| Lymphoid tissue hypoplasia | 77 | 1,319.1 954.7–1,822.7ROR 1,325.0 · χ² 47,669 / p <1e-300 | 543.3 EB05 448.9 · IC025 +6.6 3/3 methods agree | Likely indication confounding shares a disease stem with the indications wording | serious n=77 | no trial arm names this drug |
| Thymus hypoplasia | 82 | 1,117.4 827.4–1,509.1ROR 1,122.8 · χ² 46,771 / p <1e-300 | 509.7 EB05 423.6 · IC025 +6.6 3/3 methods agree | Not in label — unassessed | serious n=82 | no trial arm names this drug |
| Picornavirus infection | 14 | 699.5 361.9–1,352.1ROR 700.1 · χ² 5,734 / p <1e-300 | 275.8 EB05 174.6 · IC025 +3.9 3/3 methods agree | Not in label — unassessed | serious n=14 | no trial arm names this drug |
| Occupational asthma | 19 | 584.2 337.7–1,010.7ROR 584.9 · χ² 7,051 / p <1e-300 | 281.7 EB05 190.6 · IC025 +4.4 3/3 methods agree | Not in label — unassessed | serious n=19 | no trial arm names this drug |
| Perineal necrosis | 13 | 577.4 298.0–1,118.8ROR 577.8 · χ² 4,667 / p <1e-300 | 248.1 EB05 154.3 · IC025 +3.7 3/3 methods agree | Not in label — unassessed | serious n=13 | no trial arm names this drug |
| Cystitis bacterial | 107 | 560.3 445.6–704.5ROR 563.8 · χ² 40,336 / p <1e-300 | 357.7 EB05 304.3 · IC025 +6.8 3/3 methods agree | Not in label — unassessed | serious n=107 | no trial arm names this drug |
| Norovirus test positive | 100 | 545.1 430.5–690.1ROR 548.3 · χ² 36,965 / p <1e-300 | 349.9 EB05 296.0 · IC025 +6.7 3/3 methods agree | Lab value / disease marker | serious n=100 | no trial arm names this drug |
| Mycobacterium chelonae infection | 165 | 352.1 296.2–418.5ROR 355.5 · χ² 44,384 / p <1e-300 | 264.1 EB05 231.9 · IC025 +7.0 3/3 methods agree | Not in label — unassessed | serious n=165 | no trial arm names this drug |
| Secondary immunodeficiency | 244 | 312.9 272.0–360.0ROR 317.4 · χ² 59,926 / p <1e-300 | 243.7 EB05 219.1 · IC025 +7.2 3/3 methods agree | Labeled (related wording) related wording | serious n=244 · ♀ 100% (drug 48%) | no trial arm names this drug |
| Hepatosplenic t-cell lymphoma | 71 | 286.7 221.4–371.2ROR 287.9 · χ² 16,083 / p <1e-300 | 218.0 EB05 178.7 · IC025 +6.1 3/3 methods agree | Labeled adverse reaction exact term | serious n=71 · ♀ 22% (drug 48%) · 94% clinician | no trial arm names this drug |
| Abscess bacterial | 68 | 286.1 219.7–372.6ROR 287.3 · χ² 15,370 / p <1e-300 | 217.1 EB05 177.2 · IC025 +6.0 3/3 methods agree | Not in label — unassessed | serious n=68 | no trial arm names this drug |
| Fibrinous bronchitis | 14 | 284.6 158.9–509.5ROR 284.8 · χ² 2,972 / p <1e-300 | 175.5 EB05 111.1 · IC025 +3.8 3/3 methods agree | Not in label — unassessed | serious n=14 | no trial arm names this drug |
| Enterovirus test positive | 26 | 213.6 140.7–324.0ROR 213.9 · χ² 4,490 / p <1e-300 | 160.3 EB05 114.9 · IC025 +4.7 3/3 methods agree | Lab value / disease marker | serious n=24 · 100% clinician | no trial arm names this drug |
| Corona virus infection | 62 | 202.0 154.4–264.3ROR 202.8 · χ² 10,443 / p <1e-300 | 164.4 EB05 132.8 · IC025 +5.8 3/3 methods agree | Not in label — unassessed | serious n=62 | no trial arm names this drug |
| Treatment related secondary malignancy | 24 | 160.8 105.0–246.1ROR 161.0 · χ² 3,221 / p <1e-300 | 127.7 EB05 90.3 · IC025 +4.5 3/3 methods agree | Not in label — unassessed | serious n=23 | no trial arm names this drug |
| Parainfluenzae virus infection | 169 | 149.1 127.2–174.9ROR 150.6 · χ² 21,986 / p <1e-300 | 130.8 EB05 115.1 · IC025 +6.3 3/3 methods agree | Not in label — unassessed | serious n=169 · ♀ 23% (drug 48%) | no trial arm names this drug |
| Glioma | 66 | 139.6 108.2–180.0ROR 140.1 · χ² 8,011 / p <1e-300 | 120.8 EB05 98.3 · IC025 +5.6 3/3 methods agree | Not in label — unassessed | serious n=65 | no trial arm names this drug |
| Central hypothyroidism | 31 | 135.7 93.6–196.6ROR 135.9 · χ² 3,603 / p <1e-300 | 113.6 EB05 83.8 · IC025 +4.8 3/3 methods agree | Not in label — unassessed | serious n=31 · ♀ 74% (drug 48%) | no trial arm names this drug |
| Pseudomonal sepsis | 175 | 134.2 114.9–156.8ROR 135.5 · χ² 20,687 / p <1e-300 | 119.2 EB05 105.1 · IC025 +6.2 3/3 methods agree | Not in label — unassessed | serious n=175 | no trial arm names this drug |
| Eosinophilic pneumonia chronic | 27 | 119.9 80.8–178.1ROR 120.1 · χ² 2,787 / p <1e-300 | 101.5 EB05 73.2 · IC025 +4.6 3/3 methods agree | Not in label — unassessed | serious n=27 · ♀ 0% (drug 48%) · 70% consumer | no trial arm names this drug |
| Human herpesvirus 6 infection | 163 | 113.8 97.0–133.5ROR 114.9 · χ² 16,541 / p <1e-300 | 102.8 EB05 90.2 · IC025 +6.0 3/3 methods agree | Not in label — unassessed | serious n=163 · ♀ 100% (drug 48%) | no trial arm names this drug |
| Bacterial vulvovaginitis | 24 | 111.6 73.4–169.4ROR 111.7 · χ² 2,306 / p <1e-300 | 94.7 EB05 66.9 · IC025 +4.4 3/3 methods agree | Not in label — unassessed | serious n=24 · ♀ 100% (drug 48%) | no trial arm names this drug |
| Eosinophilic bronchitis | 17 | 111.4 67.8–183.1ROR 111.5 · χ² 1,602 / p <1e-300 | 91.8 EB05 60.7 · IC025 +3.9 3/3 methods agree | Not in label — unassessed | serious n=17 | no trial arm names this drug |
| Enterobacter infection | 138 | 108.9 91.6–129.6ROR 109.8 · χ² 13,433 / p <1e-300 | 98.6 EB05 85.6 · IC025 +5.9 3/3 methods agree | Not in label — unassessed | serious n=138 · ♀ 87% (drug 48%) | no trial arm names this drug |
| Human rhinovirus test positive | 29 | 102.6 70.2–149.8ROR 102.8 · χ² 2,595 / p <1e-300 | 89.2 EB05 65.1 · IC025 +4.6 3/3 methods agree | Lab value / disease marker | serious n=27 · 100% clinician | no trial arm names this drug |
| Temporal lobe epilepsy | 34 | 101.7 71.7–144.3ROR 101.9 · χ² 3,032 / p <1e-300 | 89.2 EB05 66.8 · IC025 +4.8 3/3 methods agree | Not in label — unassessed | serious n=34 · ♀ 100% (drug 48%) | no trial arm names this drug |
| Cytomegalovirus chorioretinitis | 134 | 90.8 76.2–108.1ROR 91.5 · χ² 10,979 / p <1e-300 | 83.5 EB05 72.3 · IC025 +5.7 3/3 methods agree | Not in label — unassessed | serious n=134 · ♀ 13% (drug 48%) | no trial arm names this drug |
| Viraemia | 96 | 90.1 73.2–110.8ROR 90.6 · χ² 7,784 / p <1e-300 | 82.5 EB05 69.6 · IC025 +5.5 3/3 methods agree | Not in label — unassessed | serious n=95 | no trial arm names this drug |
| Disseminated aspergillosis | 32 | 89.0 62.2–127.5ROR 89.2 · χ² 2,512 / p <1e-300 | 79.2 EB05 58.7 · IC025 +4.6 3/3 methods agree | Not in label — unassessed | serious n=32 | no trial arm names this drug |
| Perineal infection | 13 | 84.3 48.0–147.8ROR 84.3 · χ² 923 / p 5e-203 | 70.9 EB05 44.1 · IC025 +3.4 3/3 methods agree | Not in label — unassessed | serious n=13 | no trial arm names this drug |
| Diffuse alveolar damage | 72 | 80.9 63.8–102.7ROR 81.3 · χ² 5,250 / p <1e-300 | 74.4 EB05 61.1 · IC025 +5.3 3/3 methods agree | Not in label — unassessed | serious n=72 | no trial arm names this drug |
| T-cell type acute leukaemia | 12 | 75.3 42.1–135.0ROR 75.4 · χ² 760 / p 2e-167 | 64.1 EB05 39.0 · IC025 +3.2 3/3 methods agree | Likely indication confounding shares a disease stem with the indications wording | serious n=12 | no trial arm names this drug |
| Enterovirus infection | 47 | 70.7 52.7–94.9ROR 70.9 · χ² 2,985 / p <1e-300 | 65.1 EB05 51.0 · IC025 +4.8 3/3 methods agree | Not in label — unassessed | serious n=47 · 100% clinician | no trial arm names this drug |
| Rhinovirus infection | 202 | 68.3 59.3–78.6ROR 69.1 · χ² 12,608 / p <1e-300 | 64.3 EB05 57.2 · IC025 +5.6 3/3 methods agree | Not in label — unassessed | serious n=202 | no trial arm names this drug |
| Bacillus infection | 33 | 67.8 47.7–96.2ROR 67.9 · χ² 1,992 / p <1e-300 | 62.0 EB05 46.2 · IC025 +4.5 3/3 methods agree | Not in label — unassessed | serious n=33 · ♀ 100% (drug 48%) | no trial arm names this drug |
| Glioblastoma multiforme | 44 | 66.5 49.1–90.1ROR 66.7 · χ² 2,629 / p <1e-300 | 61.5 EB05 47.7 · IC025 +4.7 3/3 methods agree | Not in label — unassessed | serious n=44 · 91% clinician | no trial arm names this drug |
| Ileocaecal resection | 17 | 65.6 40.2–106.8ROR 65.6 · χ² 964 / p 5e-212 | 58.3 EB05 38.6 · IC025 +3.7 3/3 methods agree | Not in label — unassessed | serious n=17 | no trial arm names this drug |
| Pneumatosis intestinalis | 142 | 64.0 54.1–75.7ROR 64.5 · χ² 8,304 / p <1e-300 | 60.4 EB05 52.5 · IC025 +5.4 3/3 methods agree | Not in label — unassessed | serious n=142 · ♀ 85% (drug 48%) | no trial arm names this drug |
Trial evidence (ClinicalTrials.gov)
ClinicalTrials.gov returned results-bearing trials mentioning Mercaptopurine only as a background or rescue medication (NCT01190930, NCT02883049); no trial listing it as the tested intervention was cached, so no trial cross-check is shown.
Most frequently reported reactions
By raw co-report count, regardless of disproportionality. Common terms here are often the illness itself or generic outcomes.
| Reaction | Reports | PRR 95% CI · ROR · χ²/p | EBGM EB05 · IC025 | Label screen | Serious-only & strata | Trial cross-check |
|---|---|---|---|---|---|---|
| Febrile neutropenia | 1,903 | 36.0 34.5–37.6ROR 40.3 · χ² 62,974 / p <1e-300 | 34.9 EB05 33.6 · IC025 +5.0 3/3 methods agree | Labeled (related wording) related wording | serious PRR 23.2 · 95% clinician | no trial arm names this drug |
| Pyrexia (fever) | 1,188 | 4.2 4.0–4.4ROR 4.4 · χ² 2,914 / p <1e-300 | 4.2 EB05 4.0 · IC025 +2.0 3/3 methods agree | Labeled (related wording) related wording | serious PRR 3.3 | no trial arm names this drug |
| Off label use (used outside approved label) | 1,131 | 1.6 1.5–1.7ROR 1.6 · χ² 254 / p 1e-57 | 1.6 EB05 1.5 · IC025 +0.6 | Reporting / coding artifact | serious PRR 1.8 | no trial arm names this drug |
| Drug ineffective (drug did not work) | 1,103 | 1.0 1.0–1.1ROR 1.0 · χ² 0 / p 0.265 | 1.0 EB05 1.0 · IC025 -0.1 PRR-only: not supported by EBGM or IC | Reporting / coding artifact | serious PRR 1.6 | no trial arm names this drug |
| Nausea | 791 | 1.2 1.1–1.3ROR 1.2 · χ² 32 / p 7e-9 | 1.2 EB05 1.1 · IC025 +0.2 | Labeled adverse reaction exact term | serious PRR 1.3 | no trial arm names this drug |
| Vomiting | 749 | 1.9 1.8–2.1ROR 2.0 · χ² 350 / p 2e-78 | 1.9 EB05 1.8 · IC025 +0.8 | Labeled adverse reaction exact term | serious PRR 1.8 | no trial arm names this drug |
| Diarrhoea (diarrhea) | 700 | 1.3 1.2–1.4ROR 1.3 · χ² 57 / p 2e-14 | 1.3 EB05 1.2 · IC025 +0.3 | Labeled adverse reaction exact term | serious PRR 1.3 | no trial arm names this drug |
| Abdominal pain | 698 | 3.7 3.4–3.9ROR 3.8 · χ² 1,357 / p 2e-297 | 3.6 EB05 3.4 · IC025 +1.7 3/3 methods agree | Not in label — unassessed | serious PRR 2.8 | no trial arm names this drug |
| Headache | 602 | 1.2 1.1–1.3ROR 1.2 · χ² 15 / p 5e-5 | 1.2 EB05 1.1 · IC025 +0.1 | Not in label — unassessed | serious PRR 1.3 | no trial arm names this drug |
| Neutropenia (low neutrophils (infection-fighting cells)) | 596 | 5.4 5.0–5.8ROR 5.5 · χ² 2,119 / p <1e-300 | 5.3 EB05 5.0 · IC025 +2.3 3/3 methods agree | Labeled adverse reaction exact term | serious PRR 3.6 · 98% clinician | no trial arm names this drug |
| Fatigue (tiredness) | 554 | 0.9 0.8–0.9ROR 0.9 · χ² 12 / p 3e-4 | 0.9 EB05 0.8 · IC025 -0.3 PRR-only: not supported by EBGM or IC | Not in label — unassessed | serious PRR 0.8 | no trial arm names this drug |
| Pneumonia (lung infection) | 523 | 2.0 1.8–2.1ROR 2.0 · χ² 252 / p 5e-57 | 2.0 EB05 1.8 · IC025 +0.8 | Labeled (related wording) related wording | serious PRR 1.3 | no trial arm names this drug |
| Sepsis (blood infection) | 491 | 5.4 4.9–5.9ROR 5.5 · χ² 1,743 / p <1e-300 | 5.3 EB05 4.9 · IC025 +2.3 3/3 methods agree | Not in label — unassessed | serious PRR 3.5 · 94% clinician | no trial arm names this drug |
| Anaemia (low red blood cells) | 469 | 3.0 2.7–3.3ROR 3.0 · χ² 625 / p 3e-138 | 3.0 EB05 2.8 · IC025 +1.4 3/3 methods agree | Labeled adverse reaction exact term | serious PRR 2.1 | no trial arm names this drug |
| Condition aggravated | 451 | 1.8 1.7–2.0ROR 1.8 · χ² 167 / p 1e-38 | 1.8 EB05 1.7 · IC025 +0.7 | Reporting / coding artifact | serious PRR 1.8 | no trial arm names this drug |
Unlabeled candidate signals
These flagged terms were not found in the FDA label text and do not match an artifact pattern. They are the pairs that would merit a structured assessment (temporality, dose-response, biological plausibility, consistency across databases, trial or cohort evidence). The screen has not done that; string-matching misses label wording, and a strong PRR can still be stimulated reporting, co-medication, or the disease itself.
- Human bocavirus infection — n=130, PRR 2,687.9
- Thymus hypoplasia — n=82, PRR 1,117.4
- Cystitis bacterial — n=107, PRR 560.3
- Mycobacterium chelonae infection — n=165, PRR 352.1
- Abscess bacterial — n=68, PRR 286.1
- Corona virus infection — n=62, PRR 202.0
- Treatment related secondary malignancy — n=24, PRR 160.8
- Parainfluenzae virus infection — n=169, PRR 149.1
- Glioma — n=66, PRR 139.6
- Central hypothyroidism — n=31, PRR 135.7
Want to assess one of these properly? The runbook describes the Bradford Hill / trial-evidence workflow used on the assessment page, and handoff_top1000_signals.csv lists the strongest unassessed pairs across all drugs.
Common questions
What are the most reported side effects of Mercaptopurine?
In FDA FAERS, the most frequently co-reported reaction terms for Mercaptopurine are febrile neutropenia, pyrexia, nausea, vomiting, diarrhoea, abdominal pain. These are report counts, not rates: FAERS has no denominator, reporting is voluntary and uneven, and a term can be reported because of the illness being treated rather than the drug.
Does Mercaptopurine have an FDA boxed warning?
No. The FDA label retrieved for Mercaptopurine has no boxed warning; see the label's Warnings and Precautions for the recognised risks.
Do FAERS signals prove that Mercaptopurine causes these side effects?
No. A disproportionality signal (PRR, ROR) means a reaction is reported more often with this drug than with others in the database. That can reflect a real effect, but also the disease being treated, stimulated reporting after news or litigation, co-medications, or how reports are coded. Causality needs temporality, dose-response, biological plausibility, consistency across data sources and, ideally, trial or cohort evidence. This page screens each term against the FDA label and ClinicalTrials.gov results; it does not establish causation.
Sources & methods
- FDA label via openFDA (set ID 0a366cf3-91f0-4aa4-8f34-a7a27cad8f15, effective 20260722; matched on
openfda.generic_name.exact:"MERCAPTOPURINE") - Bayesian shrinkage measures computed offline over all 1,317,300 cached pairs: BCPNN information component (IC, with Norén closed-form IC025) and MGPS empirical-Bayes geometric mean (EBGM, EB05) with a two-component gamma prior fitted by maximum marginal likelihood (α₁=0.57, β₁=0.02, α₂=1.42, β₂=0.70, w=0.17); unstratified
- FAERS pair statistics from the OpenPV build of 2026-09-14 (openFDA
count=aggregation, fieldpatient.drug.openfda.generic_name.exact, 1,147 reaction terms, min 3 co-reports); PRR/ROR with 95% CI, chi-square with Yates correction - Serious-only and stratified counts (serious:1, patient.patientsex, primarysource.qualification) from openFDA count queries; serious-only PRR uses the drug serious report total and the term global serious total (N_serious=11,882,979); strata shown only when n ≥ 20
- Seriousness and sex counts from openFDA
count=serious,patient.patientsex,seriousnessdeath - ClinicalTrials.gov API v2 (query
MERCAPTOPURINE, results-bearing studies sorted by enrollment)
Page generated 2026-10-08. Label and PubChem text are retrieved automatically and may lag the current label; always check the current prescribing information. OpenPV is a beta research tool of the Open Source Medicine Foundation.