Hydroquinone side effects: what 583 FDA adverse event reports show

Hydroquinone · Hydro-Q · 19 reaction terms · 105 drug–reaction co-reports · data 2026-09-14

Drug class

Melanin Synthesis Inhibitor

FDA label (Established Pharmacologic Class)

Brand names

Hydro-Q

openFDA label

Who reports

583 reports · 55% serious · 19 with death outcome · 86% female

openFDA FAERS counts, all years, deduplication not applied

Used for

HYDRO-Q ® is indicated for the gradual bleaching of hyperpigmented skin conditions such as chlosma, melasma, freckles, senile lentigines and areas of melanin hyperpigmentation.

FDA label, Indications and Usage

What it is

HYDRO-Q ® contains hydroquinone USP 4%. Hydroquinone is 1, 4-Benzenediol {CAS 123-31-91}. Hydroquinone is structurally related to monobenzone. Hydroquinone occurs as fine white needles. The drug is freely soluble in water and in alcohol and has a pKa of 9.96.

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 11 strongest signals

Reporting / coding artifact 1Lab value / disease marker 1Not in label — unassessed 9

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.
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. 8 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
Application site pain	12	19.6 11.2–34.3ROR 20.0 · χ² 194 / p 2e-44	18.7 EB05 10.8 · IC025 +2.5 3/3 methods agree	Not in label — unassessed	serious n=6	no trial data cached
Hair texture abnormal	6	16.0 7.2–35.5ROR 16.2 · χ² 70 / p 3e-17	12.5 EB05 4.7 · IC025 +1.5 3/3 methods agree	Not in label — unassessed	serious n=5	no trial data cached
Tooth infection	5	13.6 5.7–32.6ROR 13.7 · χ² 47 / p 4e-12	9.2 EB05 3.4 · IC025 +1.1 3/3 methods agree	Not in label — unassessed	serious PRR 23.5	no trial data cached
Application site rash	5	13.0 5.4–31.2ROR 13.1 · χ² 44 / p 1e-11	8.7 EB05 3.3 · IC025 +1.1 3/3 methods agree	Not in label — unassessed	–	no trial data cached
Impaired quality of life	5	12.5 5.2–29.9ROR 12.6 · χ² 42 / p 4e-11	8.2 EB05 3.1 · IC025 +1.0 3/3 methods agree	Not in label — unassessed	serious PRR 11.6	no trial data cached
Incorrect drug administration duration	8	10.1 5.1–20.1ROR 10.2 · χ² 57 / p 2e-14	7.5 EB05 3.7 · IC025 +1.5 3/3 methods agree	Reporting / coding artifact	serious PRR 29.6	no trial data cached
Application site erythema	5	6.8 2.9–16.4ROR 6.9 · χ² 19 / p 5e-6	4.5 EB05 2.1 · IC025 +0.6 3/3 methods agree	Not in label — unassessed	serious n=2	no trial data cached
Bone density decreased	6	6.8 3.1–15.1ROR 6.9 · χ² 24 / p 4e-7	4.8 EB05 2.4 · IC025 +0.8 3/3 methods agree	Lab value / disease marker	serious PRR 5.9	no trial data cached
Acne	14	6.6 3.9–11.1ROR 6.7 · χ² 61 / p 2e-15	5.6 EB05 3.5 · IC025 +1.6 3/3 methods agree	Not in label — unassessed	serious PRR 21.7	no trial data cached
Skin burning sensation	9	4.6 2.4–8.7ROR 4.6 · χ² 22 / p 2e-6	3.8 EB05 2.2 · IC025 +0.8 3/3 methods agree	Not in label — unassessed	serious PRR 20.3	no trial data cached
Dermatitis atopic	5	3.6 1.5–8.6ROR 3.6 · χ² 7 / p 0.004	2.9 EB05 1.4 · IC025 -0.0 PRR-only: not supported by EBGM or IC	Not in label — unassessed	serious n=1	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
Acne	14	6.6 3.9–11.1ROR 6.7 · χ² 61 / p 2e-15	5.6 EB05 3.5 · IC025 +1.6 3/3 methods agree	Not in label — unassessed	serious PRR 21.7	no trial data cached
Application site pain	12	19.6 11.2–34.3ROR 20.0 · χ² 194 / p 2e-44	18.7 EB05 10.8 · IC025 +2.5 3/3 methods agree	Not in label — unassessed	serious n=6	no trial data cached
Skin burning sensation	9	4.6 2.4–8.7ROR 4.6 · χ² 22 / p 2e-6	3.8 EB05 2.2 · IC025 +0.8 3/3 methods agree	Not in label — unassessed	serious PRR 20.3	no trial data cached
Incorrect drug administration duration	8	10.1 5.1–20.1ROR 10.2 · χ² 57 / p 2e-14	7.5 EB05 3.7 · IC025 +1.5 3/3 methods agree	Reporting / coding artifact	serious PRR 29.6	no trial data cached
Bone density decreased	6	6.8 3.1–15.1ROR 6.9 · χ² 24 / p 4e-7	4.8 EB05 2.4 · IC025 +0.8 3/3 methods agree	Lab value / disease marker	serious PRR 5.9	no trial data cached
Hair texture abnormal	6	16.0 7.2–35.5ROR 16.2 · χ² 70 / p 3e-17	12.5 EB05 4.7 · IC025 +1.5 3/3 methods agree	Not in label — unassessed	serious n=5	no trial data cached
Dermatitis atopic	5	3.6 1.5–8.6ROR 3.6 · χ² 7 / p 0.004	2.9 EB05 1.4 · IC025 -0.0 PRR-only: not supported by EBGM or IC	Not in label — unassessed	serious n=1	no trial data cached
Application site erythema	5	6.8 2.9–16.4ROR 6.9 · χ² 19 / p 5e-6	4.5 EB05 2.1 · IC025 +0.6 3/3 methods agree	Not in label — unassessed	serious n=2	no trial data cached
Impaired quality of life	5	12.5 5.2–29.9ROR 12.6 · χ² 42 / p 4e-11	8.2 EB05 3.1 · IC025 +1.0 3/3 methods agree	Not in label — unassessed	serious PRR 11.6	no trial data cached
Application site rash	5	13.0 5.4–31.2ROR 13.1 · χ² 44 / p 1e-11	8.7 EB05 3.3 · IC025 +1.1 3/3 methods agree	Not in label — unassessed	–	no trial data cached
Tooth infection	5	13.6 5.7–32.6ROR 13.7 · χ² 47 / p 4e-12	9.2 EB05 3.4 · IC025 +1.1 3/3 methods agree	Not in label — unassessed	serious PRR 23.5	no trial data cached
Metastases to lung	4	12.5 4.7–33.1ROR 12.5 · χ² 32 / p 1e-8	7.4 EB05 2.6 · IC025 +0.7 3/3 methods agree	Not in label — unassessed	serious PRR 13.1	no trial data cached
Incorrect dose administered by device	3	4.2 1.4–13.1ROR 4.3 · χ² 5 / p 0.016	2.9 EB05 1.2 · IC025 -0.5 PRR-only: not supported by EBGM or IC	Reporting / coding artifact	–	no trial data cached
Exposure via skin contact	3	4.8 1.5–14.8ROR 4.8 · χ² 6 / p 0.009	3.1 EB05 1.2 · IC025 -0.4 PRR-only: not supported by EBGM or IC	Reporting / coding artifact	–	no trial data cached
Product preparation error	3	5.2 1.7–16.1ROR 5.2 · χ² 6 / p 0.006	3.3 EB05 1.3 · IC025 -0.4 PRR-only: not supported by EBGM or IC	Reporting / coding artifact	–	no trial data cached

Unlabeled candidate signals

Every screened signal with n ≥ 20 was either labeled, an indication term, or an artifact pattern — no unlabeled candidate stood out in the top signals.

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 Hydroquinone?

In FDA FAERS, the most frequently co-reported reaction terms for Hydroquinone are acne, application site pain, skin burning sensation, bone density decreased, hair texture abnormal, dermatitis atopic. 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 Hydroquinone have an FDA boxed warning?

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

Do FAERS signals prove that Hydroquinone 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 486781be-498a-49b4-9a80-2dfd02f5e358, effective 20230112; matched on openfda.generic_name.exact:"HYDROQUINONE")
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, 19 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.