Methimazole side effects: what 7,802 FDA adverse event reports show

Methimazole · 1,072 reaction terms · 29,371 drug–reaction co-reports · data 2026-09-14

Drug class

Thyroid Hormone Synthesis Inhibitor

FDA label (Established Pharmacologic Class)

Who reports

7,802 reports · 71% serious · 641 with death outcome · 72% female

openFDA FAERS counts, all years, deduplication not applied

Used for

Methimazole tablets are indicated: In patients with Graves' disease with hyperthyroidism or toxic multinodular goiter for whom surgery or radioactive iodine therapy is not an appropriate treatment option. To ameliorate symptoms of hyperthyroidism in preparation for thyroidectomy or radioactive iodine therapy.

FDA label, Indications and Usage

What it is

DESCRIPTION Methimazole (1-methylimidazole-2-thiol) is a white, crystalline substance that is freely soluble in water. It differs chemically from the drugs of the thiouracil series primarily because it has a 5- instead of a 6-membered ring.

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 4Labeled (related wording) 2Likely indication confounding 4Reporting / coding artifact 1Lab value / disease marker 6Not in label — unassessed 23

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.
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.
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. 1,032 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
Antithyroid arthritis syndrome	25	33,127.6 7,847.8–139,839.6ROR 33,234.1 · χ² 58,914 / p <1e-300	848.2 EB05 604.2 · IC025 +5.0 3/3 methods agree	Not in label — unassessed	serious n=25 · 100% clinician	no trial data cached
Thyrotoxic myopathy	11	14,576.2 3,231.3–65,751.8ROR 14,596.7 · χ² 22,472 / p <1e-300	456.2 EB05 271.8 · IC025 +3.5 3/3 methods agree	Not in label — unassessed	serious n=11	no trial data cached
Thyroid stimulating immunoglobulin increased	20	929.9 559.0–1,546.9ROR 932.3 · χ² 13,059 / p <1e-300	414.6 EB05 283.4 · IC025 +4.5 3/3 methods agree	Lab value / disease marker	serious n=19	no trial data cached
Thyrotoxic periodic paralysis	29	732.0 485.5–1,103.6ROR 734.7 · χ² 16,020 / p <1e-300	415.8 EB05 303.6 · IC025 +5.1 3/3 methods agree	Not in label — unassessed	serious n=29 · ♀ 0% (drug 72%) · 100% clinician	no trial data cached
Deafness permanent	40	683.9 483.5–967.5ROR 687.4 · χ² 21,142 / p <1e-300	431.2 EB05 330.3 · IC025 +5.6 3/3 methods agree	Not in label — unassessed	serious n=40	no trial data cached
Toxic goitre	10	616.3 309.8–1,226.0ROR 617.1 · χ² 4,497 / p <1e-300	256.0 EB05 148.6 · IC025 +3.3 3/3 methods agree	Not in label — unassessed	serious n=10	no trial data cached
Choanal atresia	16	407.7 241.0–689.8ROR 408.6 · χ² 5,279 / p <1e-300	248.7 EB05 162.3 · IC025 +4.1 3/3 methods agree	Labeled adverse reaction exact term	serious n=16	no trial data cached
Thyrotoxic crisis	138	374.0 313.3–446.3ROR 380.7 · χ² 44,659 / p <1e-300	313.6 EB05 272.1 · IC025 +7.0 3/3 methods agree	Not in label — unassessed	serious n=138 · 93% clinician	no trial data cached
Endocrine ophthalmopathy	81	292.1 232.4–367.0ROR 295.1 · χ² 20,903 / p <1e-300	247.8 EB05 205.7 · IC025 +6.3 3/3 methods agree	Not in label — unassessed	serious n=81	no trial data cached
Toxic nodular goitre	11	253.5 136.6–470.4ROR 253.9 · χ² 2,300 / p <1e-300	165.6 EB05 98.7 · IC025 +3.4 3/3 methods agree	Not in label — unassessed	serious n=11	no trial data cached
Anti-thyroid antibody increased	10	250.0 130.8–478.0ROR 250.3 · χ² 2,045 / p <1e-300	159.6 EB05 92.7 · IC025 +3.2 3/3 methods agree	Lab value / disease marker	serious n=10	no trial data cached
Aplasia cutis congenita	10	205.4 108.0–390.8ROR 205.7 · χ² 1,703 / p <1e-300	140.3 EB05 81.5 · IC025 +3.2 3/3 methods agree	Labeled (related wording) related wording	serious n=10	no trial data cached
Post procedural hypothyroidism	10	135.9 72.0–256.5ROR 136.1 · χ² 1,149 / p 4e-252	104.2 EB05 60.5 · IC025 +3.1 3/3 methods agree	Reporting / coding artifact	serious n=10	no trial data cached
Insulin autoimmune syndrome	12	97.3 54.7–173.0ROR 97.4 · χ² 1,012 / p 2e-222	82.0 EB05 50.0 · IC025 +3.3 3/3 methods agree	Labeled adverse reaction exact term	serious n=12	no trial data cached
Hyperthyroidism	407	77.8 70.6–85.6ROR 82.0 · χ² 29,909 / p <1e-300	75.3 EB05 69.4 · IC025 +5.9 3/3 methods agree	Likely indication confounding term in indications; also in warnings	serious PRR 63.3	no trial data cached
Tri-iodothyronine increased	10	69.0 36.9–129.3ROR 69.1 · χ² 589 / p 2e-130	60.0 EB05 34.8 · IC025 +2.9 3/3 methods agree	Not in label — unassessed	serious n=10	no trial data cached
Thyroxine free increased	16	68.4 41.7–112.3ROR 68.5 · χ² 971 / p 2e-213	62.0 EB05 40.5 · IC025 +3.6 3/3 methods agree	Lab value / disease marker	serious n=14	no trial data cached
Exomphalos	12	60.8 34.3–107.7ROR 60.9 · χ² 633 / p 6e-140	54.6 EB05 33.3 · IC025 +3.2 3/3 methods agree	Not in label — unassessed	serious n=12	no trial data cached
Deafness neurosensory	56	46.7 35.9–60.7ROR 47.0 · χ² 2,416 / p <1e-300	45.2 EB05 36.1 · IC025 +4.6 3/3 methods agree	Not in label — unassessed	serious n=56 · 100% clinician	no trial data cached
Lip erosion	12	44.5 25.2–78.7ROR 44.5 · χ² 460 / p 3e-102	41.1 EB05 25.1 · IC025 +3.0 3/3 methods agree	Not in label — unassessed	serious n=12	no trial data cached
Exophthalmos	34	43.6 31.1–61.1ROR 43.8 · χ² 1,351 / p 6e-296	42.0 EB05 31.4 · IC025 +4.2 3/3 methods agree	Not in label — unassessed	serious n=30	no trial data cached
Agranulocytosis (severe loss of white cells)	259	38.2 33.8–43.1ROR 39.4 · χ² 9,212 / p <1e-300	37.5 EB05 33.9 · IC025 +4.9 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 30.5 · 99% clinician	no trial data cached
Antineutrophil cytoplasmic antibody positive	10	37.4 20.0–69.7ROR 37.4 · χ² 314 / p 1e-70	34.6 EB05 20.1 · IC025 +2.7 3/3 methods agree	Lab value / disease marker	serious n=10	no trial data cached
Goitre	50	33.1 25.0–43.7ROR 33.3 · χ² 1,505 / p <1e-300	32.3 EB05 25.5 · IC025 +4.2 3/3 methods agree	Not in label — unassessed	serious n=45	no trial data cached
Anti-neutrophil cytoplasmic antibody positive vasculitis	25	27.8 18.8–41.2ROR 27.9 · χ² 614 / p 9e-136	27.0 EB05 19.3 · IC025 +3.5 3/3 methods agree	Labeled (related wording) related wording	serious n=25 · 100% clinician	no trial data cached
Jaundice cholestatic	36	27.6 19.9–38.3ROR 27.7 · χ² 887 / p 4e-195	27.0 EB05 20.3 · IC025 +3.8 3/3 methods agree	Not in label — unassessed	serious n=36 · 100% clinician	no trial data cached
Ischaemic hepatitis	13	26.1 15.1–45.0ROR 26.1 · χ² 286 / p 2e-64	24.9 EB05 15.4 · IC025 +2.8 3/3 methods agree	Not in label — unassessed	serious n=13	no trial data cached
Pulseless electrical activity	35	22.7 16.3–31.7ROR 22.8 · χ² 699 / p 2e-154	22.3 EB05 16.7 · IC025 +3.5 3/3 methods agree	Not in label — unassessed	serious n=35 · ♀ 43% (drug 72%) · 100% clinician	no trial data cached
Dysmorphism	18	20.5 12.9–32.5ROR 20.5 · χ² 312 / p 5e-70	19.8 EB05 13.1 · IC025 +2.9 3/3 methods agree	Labeled adverse reaction exact term	serious n=18	no trial data cached
Physical disability	33	20.0 14.2–28.2ROR 20.1 · χ² 574 / p 4e-127	19.7 EB05 14.6 · IC025 +3.4 3/3 methods agree	Not in label — unassessed	serious n=33 · 100% clinician	no trial data cached
Blood thyroid stimulating hormone decreased	32	19.0 13.4–26.9ROR 19.1 · χ² 525 / p 2e-116	18.7 EB05 13.8 · IC025 +3.3 3/3 methods agree	Lab value / disease marker	serious n=27	no trial data cached
Cholestatic liver injury	15	18.9 11.4–31.4ROR 19.0 · χ² 235 / p 2e-53	18.2 EB05 11.3 · IC025 +2.7 3/3 methods agree	Not in label — unassessed	serious n=14	no trial data cached
Thyroiditis	20	18.1 11.7–28.2ROR 18.2 · χ² 305 / p 1e-68	17.6 EB05 11.8 · IC025 +2.9 3/3 methods agree	Likely indication confounding shares a disease stem with the indications wording	serious n=20	no trial data cached
Mean cell volume decreased	10	17.5 9.4–32.5ROR 17.5 · χ² 138 / p 3e-32	16.0 EB05 8.1 · IC025 +2.2 3/3 methods agree	Lab value / disease marker	serious n=10	no trial data cached
Thyroid mass	24	17.1 11.4–25.5ROR 17.1 · χ² 345 / p 2e-77	16.6 EB05 11.6 · IC025 +3.0 3/3 methods agree	Likely indication confounding shares a disease stem with the indications wording	serious n=22	no trial data cached
Thyroidectomy	10	15.7 8.5–29.3ROR 15.8 · χ² 123 / p 7e-29	14.0 EB05 7.0 · IC025 +2.1 3/3 methods agree	Likely indication confounding term in indications	serious n=10	no trial data cached
Polyarthritis	29	14.9 10.4–21.5ROR 15.0 · χ² 361 / p 1e-80	14.5 EB05 10.4 · IC025 +3.0 3/3 methods agree	Not in label — unassessed	serious n=29 · 100% clinician	no trial data cached
Concomitant disease aggravated	34	14.9 10.6–20.8ROR 14.9 · χ² 424 / p 2e-94	14.5 EB05 10.7 · IC025 +3.1 3/3 methods agree	Not in label — unassessed	serious n=33 · 100% clinician	no trial data cached
Ventricular dysfunction	13	13.5 7.8–23.3ROR 13.5 · χ² 138 / p 4e-32	12.1 EB05 6.8 · IC025 +2.3 3/3 methods agree	Not in label — unassessed	serious n=13	no trial data cached
Respiratory syncytial virus infection	43	13.4 9.9–18.1ROR 13.5 · χ² 479 / p 2e-106	13.1 EB05 10.0 · IC025 +3.0 3/3 methods agree	Not in label — unassessed	serious PRR 12.7 · ♀ 100% (drug 72%)	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
Drug ineffective (drug did not work)	644	1.3 1.2–1.4ROR 1.3 · χ² 51 / p 4e-13	1.3 EB05 1.2 · IC025 +0.3	Reporting / coding artifact	serious PRR 2.2	no trial data cached
Off label use (used outside approved label)	487	1.5 1.4–1.6ROR 1.5 · χ² 86 / p 8e-21	1.5 EB05 1.4 · IC025 +0.4	Reporting / coding artifact	serious PRR 1.4	no trial data cached
Fatigue (tiredness)	445	1.5 1.4–1.7ROR 1.6 · χ² 87 / p 6e-21	1.5 EB05 1.4 · IC025 +0.5	Not in label — unassessed	serious PRR 1.5	no trial data cached
Hyperthyroidism	407	77.8 70.6–85.6ROR 82.0 · χ² 29,909 / p <1e-300	75.3 EB05 69.4 · IC025 +5.9 3/3 methods agree	Likely indication confounding term in indications; also in warnings	serious PRR 63.3	no trial data cached
Nausea	381	1.3 1.2–1.4ROR 1.3 · χ² 27 / p 1e-7	1.3 EB05 1.2 · IC025 +0.2	Labeled adverse reaction exact term	serious PRR 1.2	no trial data cached
Diarrhoea (diarrhea)	335	1.4 1.3–1.6ROR 1.4 · χ² 39 / p 2e-10	1.4 EB05 1.3 · IC025 +0.3	Not in label — unassessed	serious PRR 1.3	no trial data cached
Headache	330	1.4 1.3–1.6ROR 1.4 · χ² 41 / p 7e-11	1.4 EB05 1.3 · IC025 +0.3	Labeled adverse reaction exact term	serious PRR 1.4	no trial data cached
Pain	310	1.4 1.2–1.5ROR 1.4 · χ² 29 / p 3e-8	1.4 EB05 1.2 · IC025 +0.2	Labeled adverse reaction exact term	serious PRR 1.3	no trial data cached
Dizziness	306	1.7 1.5–1.8ROR 1.7 · χ² 80 / p 2e-19	1.7 EB05 1.5 · IC025 +0.5	Not in label — unassessed	serious PRR 1.6	no trial data cached
Dyspnoea (shortness of breath)	277	1.3 1.2–1.5ROR 1.3 · χ² 21 / p 2e-6	1.3 EB05 1.2 · IC025 +0.2	Not in label — unassessed	serious PRR 1.2	no trial data cached
Agranulocytosis (severe loss of white cells)	259	38.2 33.8–43.1ROR 39.4 · χ² 9,212 / p <1e-300	37.5 EB05 33.9 · IC025 +4.9 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 30.5 · 99% clinician	no trial data cached
Asthenia (weakness)	222	1.6 1.4–1.8ROR 1.6 · χ² 48 / p 2e-12	1.6 EB05 1.4 · IC025 +0.4	Not in label — unassessed	serious PRR 1.3	no trial data cached
Arthralgia (joint pain)	221	1.4 1.3–1.6ROR 1.4 · χ² 29 / p 3e-8	1.4 EB05 1.3 · IC025 +0.3	Labeled adverse reaction exact term	serious PRR 1.5	no trial data cached
Anxiety	213	2.0 1.8–2.3ROR 2.0 · χ² 108 / p 1e-25	2.0 EB05 1.8 · IC025 +0.8	Not in label — unassessed	serious PRR 2.0	no trial data cached
Rash	212	1.3 1.2–1.5ROR 1.3 · χ² 16 / p 3e-5	1.3 EB05 1.2 · IC025 +0.2	Labeled adverse reaction exact term	serious PRR 1.3	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.

Antithyroid arthritis syndrome — n=25, PRR 33,127.6
Thyrotoxic periodic paralysis — n=29, PRR 732.0
Deafness permanent — n=40, PRR 683.9
Thyrotoxic crisis — n=138, PRR 374.0
Endocrine ophthalmopathy — n=81, PRR 292.1
Deafness neurosensory — n=56, PRR 46.7
Exophthalmos — n=34, PRR 43.6
Goitre — n=50, PRR 33.1
Jaundice cholestatic — n=36, PRR 27.6
Pulseless electrical activity — n=35, PRR 22.7

✓ A hand-written Bradford Hill causality assessment exists for this drug: Methimazole ↔ Agranulocytosis.

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 Methimazole?

In FDA FAERS, the most frequently co-reported reaction terms for Methimazole are fatigue, hyperthyroidism, nausea, diarrhoea, headache, 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 Methimazole have an FDA boxed warning?

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

Do FAERS signals prove that Methimazole 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 0f5fb0a4-feb9-4b86-9a93-b2c84cfc082f, effective 20220422; matched on openfda.generic_name.exact:"METHIMAZOLE")
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, 1,072 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.