Pyrimethamine side effects: what 1,262 FDA adverse event reports show

Pyrimethamine · 51 reaction terms · 688 drug–reaction co-reports · data 2026-09-14

Drug class

Dihydrofolate Reductase Inhibitor Antimalarial

FDA label (Established Pharmacologic Class)

Who reports

1,262 reports · 77% serious · 220 with death outcome · 45% female

openFDA FAERS counts, all years, deduplication not applied

Used for

Treatment of Toxoplasmosis : Pyrimethamine tablets are indicated for the treatment of toxoplasmosis when used conjointly with a sulfonamide, since synergism exists with this combination.

FDA label, Indications and Usage

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 25 strongest signals

Labeled adverse reaction 5Labeled (related wording) 1Reporting / coding artifact 3Not in label — unassessed 16

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.
Reporting / coding artifact An administrative, monitoring or exposure term (product issue, off-label use, 'test normal', overdose...). Not a biological reaction; a high PRR here reflects how reports are coded.
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. 26 further terms are in the interactive explorer. Reaction links open the term's cross-drug page.

Show: hide coding/lab/outcome terms hide indication terms unlabeled only hide PRR-only

Reaction	Reports	PRR 95% CI · ROR · χ²/p	EBGM EB05 · IC025	Label screen	Serious-only & strata	Trial cross-check
Clostridium difficile colitis	19	29.8 19.1–46.6ROR 30.2 · χ² 500 / p 5e-111	29.0 EB05 19.6 · IC025 +3.3 3/3 methods agree	Not in label — unassessed	serious PRR 22.7	no trial data cached
Lymphopenia	23	27.2 18.2–40.8ROR 27.7 · χ² 555 / p 6e-123	26.7 EB05 18.7 · IC025 +3.4 3/3 methods agree	Not in label — unassessed	serious PRR 21.9 · 100% clinician	no trial data cached
Bone marrow failure	33	26.9 19.2–37.6ROR 27.6 · χ² 795 / p 3e-175	26.5 EB05 19.7 · IC025 +3.7 3/3 methods agree	Not in label — unassessed	serious PRR 20.1 · ♀ 67% (drug 45%) · 97% clinician	no trial data cached
Pancytopenia (all blood cells low)	87	26.0 21.2–31.8ROR 27.9 · χ² 2,069 / p <1e-300	25.8 EB05 21.6 · IC025 +4.2 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 19.5 · 93% clinician	no trial data cached
Pneumocystis jirovecii pneumonia	11	18.3 10.2–33.0ROR 18.5 · χ² 163 / p 1e-37	17.2 EB05 9.3 · IC025 +2.4 3/3 methods agree	Not in label — unassessed	serious PRR 13.7	no trial data cached
Nephropathy	10	17.7 9.6–32.9ROR 17.9 · χ² 142 / p 6e-33	16.3 EB05 8.4 · IC025 +2.2 3/3 methods agree	Not in label — unassessed	serious PRR 14.1	no trial data cached
Drug resistance	26	16.8 11.5–24.7ROR 17.2 · χ² 372 / p 3e-83	16.5 EB05 11.7 · IC025 +3.0 3/3 methods agree	Reporting / coding artifact	serious PRR 14.1 · 96% clinician	no trial data cached
Hepatic cytolysis	12	16.3 9.3–28.7ROR 16.5 · χ² 158 / p 2e-36	15.2 EB05 8.4 · IC025 +2.4 3/3 methods agree	Not in label — unassessed	serious PRR 12.2	no trial data cached
Central nervous system lesion	13	15.6 9.1–26.9ROR 15.8 · χ² 164 / p 8e-38	14.6 EB05 8.2 · IC025 +2.4 3/3 methods agree	Not in label — unassessed	serious PRR 17.1	no trial data cached
Drug reaction with eosinophilia and systemic symptoms	23	15.0 10.0–22.5ROR 15.2 · χ² 286 / p 2e-64	14.5 EB05 9.8 · IC025 +2.8 3/3 methods agree	Not in label — unassessed	serious PRR 11.2 · ♀ 73% (drug 45%) · 100% clinician	no trial data cached
Hepatocellular injury	12	14.0 8.0–24.7ROR 14.2 · χ² 133 / p 5e-31	12.6 EB05 6.8 · IC025 +2.2 3/3 methods agree	Not in label — unassessed	serious PRR 10.5	no trial data cached
Toxic epidermal necrolysis (severe skin peeling reaction)	12	13.2 7.5–23.3ROR 13.4 · χ² 124 / p 5e-29	11.7 EB05 6.3 · IC025 +2.2 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 9.9	no trial data cached
Agranulocytosis (severe loss of white cells)	14	12.6 7.5–21.2ROR 12.7 · χ² 138 / p 3e-32	11.3 EB05 6.5 · IC025 +2.3 3/3 methods agree	Labeled (related wording) related wording	serious PRR 9.4	no trial data cached
Cholestasis (bile flow blockage)	13	11.3 6.6–19.5ROR 11.4 · χ² 112 / p 2e-26	9.8 EB05 5.5 · IC025 +2.1 3/3 methods agree	Not in label — unassessed	serious PRR 8.6	no trial data cached
Stevens-johnson syndrome (severe blistering skin reaction)	14	10.5 6.2–17.6ROR 10.6 · χ² 110 / p 4e-26	9.0 EB05 5.3 · IC025 +2.1 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 7.9	no trial data cached
Coagulopathy	10	10.0 5.4–18.6ROR 10.1 · χ² 72 / p 9e-18	7.9 EB05 4.2 · IC025 +1.7 3/3 methods agree	Not in label — unassessed	serious PRR 7.8	no trial data cached
Rash papular	13	9.6 5.6–16.5ROR 9.7 · χ² 92 / p 4e-22	8.0 EB05 4.7 · IC025 +1.9 3/3 methods agree	Not in label — unassessed	serious PRR 8.3	no trial data cached
Neuralgia	14	9.6 5.7–16.2ROR 9.7 · χ² 100 / p 1e-23	8.1 EB05 4.9 · IC025 +2.0 3/3 methods agree	Not in label — unassessed	serious PRR 10.6	no trial data cached
Leukopenia (low white blood cells)	27	9.0 6.2–13.1ROR 9.2 · χ² 185 / p 3e-42	8.2 EB05 5.8 · IC025 +2.3 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 7.0 · ♀ 67% (drug 45%)	no trial data cached
Neutropenia (low neutrophils (infection-fighting cells))	72	8.8 7.1–11.1ROR 9.3 · χ² 496 / p 4e-110	8.5 EB05 7.0 · IC025 +2.7 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 6.8 · 91% clinician	no trial data cached
Pancreatitis (pancreas inflammation)	24	7.5 5.1–11.2ROR 7.7 · χ² 130 / p 2e-30	6.8 EB05 4.7 · IC025 +2.1 3/3 methods agree	Not in label — unassessed	serious PRR 5.7 · ♀ 10% (drug 45%) · 96% clinician	no trial data cached
Premature baby	11	6.1 3.4–10.9ROR 6.1 · χ² 42 / p 6e-11	4.9 EB05 2.9 · IC025 +1.3 3/3 methods agree	Not in label — unassessed	serious PRR 4.6	no trial data cached
Visual acuity reduced	10	5.0 2.7–9.2ROR 5.0 · χ² 28 / p 6e-8	4.1 EB05 2.4 · IC025 +1.0 3/3 methods agree	Not in label — unassessed	serious PRR 5.0	no trial data cached
Maternal exposure during pregnancy	23	4.3 2.9–6.5ROR 4.4 · χ² 56 / p 4e-14	4.0 EB05 2.8 · IC025 +1.3 3/3 methods agree	Reporting / coding artifact	serious PRR 3.9 · ♀ 77% (drug 45%) · 96% clinician	no trial data cached
Foetal exposure during pregnancy	18	3.7 2.3–5.8ROR 3.7 · χ² 33 / p 5e-9	3.4 EB05 2.3 · IC025 +1.0 3/3 methods agree	Reporting / coding artifact	serious PRR 2.8	no trial data cached

Trial evidence (ClinicalTrials.gov)

Trial cross-check not yet run for this drug (ClinicalTrials.gov pull covers the 800 most-reported drugs first).

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
Pancytopenia (all blood cells low)	87	26.0 21.2–31.8ROR 27.9 · χ² 2,069 / p <1e-300	25.8 EB05 21.6 · IC025 +4.2 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 19.5 · 93% clinician	no trial data cached
Neutropenia (low neutrophils (infection-fighting cells))	72	8.8 7.1–11.1ROR 9.3 · χ² 496 / p 4e-110	8.5 EB05 7.0 · IC025 +2.7 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 6.8 · 91% clinician	no trial data cached
Bone marrow failure	33	26.9 19.2–37.6ROR 27.6 · χ² 795 / p 3e-175	26.5 EB05 19.7 · IC025 +3.7 3/3 methods agree	Not in label — unassessed	serious PRR 20.1 · ♀ 67% (drug 45%) · 97% clinician	no trial data cached
Leukopenia (low white blood cells)	27	9.0 6.2–13.1ROR 9.2 · χ² 185 / p 3e-42	8.2 EB05 5.8 · IC025 +2.3 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 7.0 · ♀ 67% (drug 45%)	no trial data cached
Drug resistance	26	16.8 11.5–24.7ROR 17.2 · χ² 372 / p 3e-83	16.5 EB05 11.7 · IC025 +3.0 3/3 methods agree	Reporting / coding artifact	serious PRR 14.1 · 96% clinician	no trial data cached
Pancreatitis (pancreas inflammation)	24	7.5 5.1–11.2ROR 7.7 · χ² 130 / p 2e-30	6.8 EB05 4.7 · IC025 +2.1 3/3 methods agree	Not in label — unassessed	serious PRR 5.7 · ♀ 10% (drug 45%) · 96% clinician	no trial data cached
Maternal exposure during pregnancy	23	4.3 2.9–6.5ROR 4.4 · χ² 56 / p 4e-14	4.0 EB05 2.8 · IC025 +1.3 3/3 methods agree	Reporting / coding artifact	serious PRR 3.9 · ♀ 77% (drug 45%) · 96% clinician	no trial data cached
Drug reaction with eosinophilia and systemic symptoms	23	15.0 10.0–22.5ROR 15.2 · χ² 286 / p 2e-64	14.5 EB05 9.8 · IC025 +2.8 3/3 methods agree	Not in label — unassessed	serious PRR 11.2 · ♀ 73% (drug 45%) · 100% clinician	no trial data cached
Lymphopenia	23	27.2 18.2–40.8ROR 27.7 · χ² 555 / p 6e-123	26.7 EB05 18.7 · IC025 +3.4 3/3 methods agree	Not in label — unassessed	serious PRR 21.9 · 100% clinician	no trial data cached
Clostridium difficile colitis	19	29.8 19.1–46.6ROR 30.2 · χ² 500 / p 5e-111	29.0 EB05 19.6 · IC025 +3.3 3/3 methods agree	Not in label — unassessed	serious PRR 22.7	no trial data cached
Foetal exposure during pregnancy	18	3.7 2.3–5.8ROR 3.7 · χ² 33 / p 5e-9	3.4 EB05 2.3 · IC025 +1.0 3/3 methods agree	Reporting / coding artifact	serious PRR 2.8	no trial data cached
Neuralgia	14	9.6 5.7–16.2ROR 9.7 · χ² 100 / p 1e-23	8.1 EB05 4.9 · IC025 +2.0 3/3 methods agree	Not in label — unassessed	serious PRR 10.6	no trial data cached
Stevens-johnson syndrome (severe blistering skin reaction)	14	10.5 6.2–17.6ROR 10.6 · χ² 110 / p 4e-26	9.0 EB05 5.3 · IC025 +2.1 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 7.9	no trial data cached
Agranulocytosis (severe loss of white cells)	14	12.6 7.5–21.2ROR 12.7 · χ² 138 / p 3e-32	11.3 EB05 6.5 · IC025 +2.3 3/3 methods agree	Labeled (related wording) related wording	serious PRR 9.4	no trial data cached
Rash papular	13	9.6 5.6–16.5ROR 9.7 · χ² 92 / p 4e-22	8.0 EB05 4.7 · IC025 +1.9 3/3 methods agree	Not in label — unassessed	serious PRR 8.3	no trial data cached

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.

Lymphopenia — n=23, PRR 27.2
Bone marrow failure — n=33, PRR 26.9
Drug reaction with eosinophilia and systemic symptoms — n=23, PRR 15.0
Pancreatitis — n=24, PRR 7.5

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 Pyrimethamine?

In FDA FAERS, the most frequently co-reported reaction terms for Pyrimethamine are pancytopenia, neutropenia, bone marrow failure, leukopenia, pancreatitis, drug reaction with eosinophilia and systemic symptoms. 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 Pyrimethamine have an FDA boxed warning?

No. The FDA label retrieved for Pyrimethamine has no boxed warning; see the label's Warnings and Precautions for the recognised risks.

Do FAERS signals prove that Pyrimethamine 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 2c57a992-619b-40ce-9395-f12ac47a2c67, effective 20210819; matched on openfda.generic_name.exact:"PYRIMETHAMINE")
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, field patient.drug.openfda.generic_name.exact, 51 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

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.