Chlorzoxazone side effects: what 1,296 FDA adverse event reports show

Chlorzoxazone · 45 reaction terms · 466 drug–reaction co-reports · data 2026-09-14

Drug class

Muscle Relaxant

FDA label (Established Pharmacologic Class) · compare the class

Who reports

1,296 reports · 65% serious · 130 with death outcome · 75% female

openFDA FAERS counts, all years, deduplication not applied

Used for

Chlorzoxazone is indicated as an adjunct to rest, physical therapy, and other measures for the relief of discomfort associated with acute, painful musculoskeletal conditions. The mode of action of this drug has not been clearly identified, but may be related to its sedative properties. Chlorzoxazone does not directly relax tense skeletal muscles in man.

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 15 strongest signals

Reporting / coding artifact 2Outcome field, not a reaction 1Not in label — unassessed 12

What the badges mean

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.
Outcome field, not a reaction Death is a report outcome, not a reaction term. It is dominated by the underlying illness; see the seriousness profile and the boxed warning instead.
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. 30 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
End stage renal disease	17	14.9 9.3–24.0ROR 15.1 · χ² 207 / p 2e-47	14.2 EB05 8.8 · IC025 +2.6 3/3 methods agree	Not in label — unassessed	serious PRR 13.3	no trial data cached
Osteopenia	13	13.8 8.0–23.7ROR 13.9 · χ² 142 / p 6e-33	12.5 EB05 7.0 · IC025 +2.3 3/3 methods agree	Not in label — unassessed	serious PRR 14.5	no trial data cached
Wrist fracture	10	13.5 7.3–25.0ROR 13.6 · χ² 104 / p 1e-24	11.6 EB05 5.8 · IC025 +2.0 3/3 methods agree	Not in label — unassessed	serious PRR 14.1	no trial data cached
Stevens-johnson syndrome (severe blistering skin reaction)	18	13.1 8.3–20.7ROR 13.3 · χ² 189 / p 3e-43	12.2 EB05 7.6 · IC025 +2.5 3/3 methods agree	Not in label — unassessed	serious PRR 11.7	no trial data cached
Chronic kidney disease	56	11.8 9.2–15.3ROR 12.3 · χ² 546 / p 5e-121	11.6 EB05 9.1 · IC025 +3.0 3/3 methods agree	Not in label — unassessed	serious PRR 10.7	no trial data cached
Sensory disturbance	11	11.0 6.1–19.9ROR 11.1 · χ² 91 / p 8e-22	9.1 EB05 4.9 · IC025 +1.9 3/3 methods agree	Not in label — unassessed	serious PRR 13.8	no trial data cached
Anhedonia	18	11.0 7.0–17.4ROR 11.1 · χ² 154 / p 1e-35	9.9 EB05 6.2 · IC025 +2.3 3/3 methods agree	Not in label — unassessed	serious PRR 10.8	no trial data cached
Economic problem	10	10.5 5.7–19.5ROR 10.6 · χ² 77 / p 1e-18	8.4 EB05 4.4 · IC025 +1.8 3/3 methods agree	Reporting / coding artifact	serious PRR 10.2	no trial data cached
Sleep apnoea syndrome	12	10.2 5.8–17.8ROR 10.2 · χ² 90 / p 1e-21	8.4 EB05 4.8 · IC025 +1.9 3/3 methods agree	Not in label — unassessed	serious PRR 10.7	no trial data cached
Renal injury	16	8.6 5.3–14.0ROR 8.7 · χ² 100 / p 6e-24	7.4 EB05 4.6 · IC025 +2.0 3/3 methods agree	Not in label — unassessed	serious PRR 7.7	no trial data cached
Completed suicide (death by suicide)	43	8.3 6.2–11.1ROR 8.5 · χ² 269 / p 1e-60	7.8 EB05 6.0 · IC025 +2.4 3/3 methods agree	Outcome field, not a reaction	serious PRR 7.3 · 100% clinician	no trial data cached
Suicide attempt	22	5.9 3.9–8.9ROR 5.9 · χ² 84 / p 2e-20	5.3 EB05 3.7 · IC025 +1.7 3/3 methods agree	Not in label — unassessed	serious PRR 5.2 · 95% clinician	no trial data cached
Intentional overdose	17	4.3 2.7–6.9ROR 4.4 · χ² 40 / p 1e-10	3.9 EB05 2.6 · IC025 +1.2 3/3 methods agree	Reporting / coding artifact	serious PRR 4.0	no trial data cached
Emotional distress	25	3.3 2.3–4.9ROR 3.4 · χ² 39 / p 3e-10	3.2 EB05 2.3 · IC025 +1.0 3/3 methods agree	Not in label — unassessed	serious PRR 3.3	no trial data cached
Drug withdrawal syndrome	11	2.0 1.1–3.6ROR 2.0 · χ² 5 / p 0.016	1.9 EB05 1.2 · IC025 -0.1 PRR-only: not supported by EBGM or IC	Not in label — unassessed	serious PRR 2.0	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
Chronic kidney disease	56	11.8 9.2–15.3ROR 12.3 · χ² 546 / p 5e-121	11.6 EB05 9.1 · IC025 +3.0 3/3 methods agree	Not in label — unassessed	serious PRR 10.7	no trial data cached
Completed suicide (death by suicide)	43	8.3 6.2–11.1ROR 8.5 · χ² 269 / p 1e-60	7.8 EB05 6.0 · IC025 +2.4 3/3 methods agree	Outcome field, not a reaction	serious PRR 7.3 · 100% clinician	no trial data cached
Emotional distress	25	3.3 2.3–4.9ROR 3.4 · χ² 39 / p 3e-10	3.2 EB05 2.3 · IC025 +1.0 3/3 methods agree	Not in label — unassessed	serious PRR 3.3	no trial data cached
Suicide attempt	22	5.9 3.9–8.9ROR 5.9 · χ² 84 / p 2e-20	5.3 EB05 3.7 · IC025 +1.7 3/3 methods agree	Not in label — unassessed	serious PRR 5.2 · 95% clinician	no trial data cached
Anhedonia	18	11.0 7.0–17.4ROR 11.1 · χ² 154 / p 1e-35	9.9 EB05 6.2 · IC025 +2.3 3/3 methods agree	Not in label — unassessed	serious PRR 10.8	no trial data cached
Stevens-johnson syndrome (severe blistering skin reaction)	18	13.1 8.3–20.7ROR 13.3 · χ² 189 / p 3e-43	12.2 EB05 7.6 · IC025 +2.5 3/3 methods agree	Not in label — unassessed	serious PRR 11.7	no trial data cached
Intentional overdose	17	4.3 2.7–6.9ROR 4.4 · χ² 40 / p 1e-10	3.9 EB05 2.6 · IC025 +1.2 3/3 methods agree	Reporting / coding artifact	serious PRR 4.0	no trial data cached
End stage renal disease	17	14.9 9.3–24.0ROR 15.1 · χ² 207 / p 2e-47	14.2 EB05 8.8 · IC025 +2.6 3/3 methods agree	Not in label — unassessed	serious PRR 13.3	no trial data cached
Renal injury	16	8.6 5.3–14.0ROR 8.7 · χ² 100 / p 6e-24	7.4 EB05 4.6 · IC025 +2.0 3/3 methods agree	Not in label — unassessed	serious PRR 7.7	no trial data cached
Osteopenia	13	13.8 8.0–23.7ROR 13.9 · χ² 142 / p 6e-33	12.5 EB05 7.0 · IC025 +2.3 3/3 methods agree	Not in label — unassessed	serious PRR 14.5	no trial data cached
Sleep apnoea syndrome	12	10.2 5.8–17.8ROR 10.2 · χ² 90 / p 1e-21	8.4 EB05 4.8 · IC025 +1.9 3/3 methods agree	Not in label — unassessed	serious PRR 10.7	no trial data cached
Drug withdrawal syndrome	11	2.0 1.1–3.6ROR 2.0 · χ² 5 / p 0.016	1.9 EB05 1.2 · IC025 -0.1 PRR-only: not supported by EBGM or IC	Not in label — unassessed	serious PRR 2.0	no trial data cached
Sensory disturbance	11	11.0 6.1–19.9ROR 11.1 · χ² 91 / p 8e-22	9.1 EB05 4.9 · IC025 +1.9 3/3 methods agree	Not in label — unassessed	serious PRR 13.8	no trial data cached
Economic problem	10	10.5 5.7–19.5ROR 10.6 · χ² 77 / p 1e-18	8.4 EB05 4.4 · IC025 +1.8 3/3 methods agree	Reporting / coding artifact	serious PRR 10.2	no trial data cached
Wrist fracture	10	13.5 7.3–25.0ROR 13.6 · χ² 104 / p 1e-24	11.6 EB05 5.8 · IC025 +2.0 3/3 methods agree	Not in label — unassessed	serious PRR 14.1	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.

Chronic kidney disease — n=56, PRR 11.8
Suicide attempt — n=22, PRR 5.9
Emotional distress — n=25, PRR 3.3

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 Chlorzoxazone?

In FDA FAERS, the most frequently co-reported reaction terms for Chlorzoxazone are chronic kidney disease, emotional distress, suicide attempt, anhedonia, stevens-johnson syndrome, end stage renal disease. 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 Chlorzoxazone have an FDA boxed warning?

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

Do FAERS signals prove that Chlorzoxazone 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 0f222059-1ece-44f3-ae7f-d277e5bcaba4, effective 20250316; matched on openfda.generic_name.exact:"CHLORZOXAZONE")
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, 45 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.