Drug · FAERS side-effect profile with label screen

Trihexyphenidyl side effects: what 995 FDA adverse event reports show

Trihexyphenidyl Hydrochloride · 45 reaction terms · 754 drug–reaction co-reports · data 2026-09-14

Drug class

Not stated in label

Label matched

Trihexyphenidyl Hydrochloride (Trihexyphenidyl Hydrochloride)

closest FDA label found for this product string; brand list suppressed because the label is a related product, not an exact match

Who reports

995 reports · 91% serious · 177 with death outcome · 44% female

openFDA FAERS counts, all years, deduplication not applied

Used for

Trihexyphenidyl is indicated as an adjunct in the treatment of all forms of parkinsonism (postencephalitic, arteriosclerotic, and idiopathic). It is often useful as adjuvant therapy when treating these forms of parkinsonism with levodopa. Additionally,.

FDA label, Indications and Usage

What it is

DESCRIPTION Trihexyphenidyl Hydrochloride Oral Solution USP is a synthetic antispasmodic drug. It is designated chemically as α-Cyclohexyl-α-phenyl-1-piperidinepropanol hydrochloride and its s.

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 32 strongest signals

Labeled adverse reaction 5Labeled (related wording) 2Likely indication confounding 2Reporting / coding artifact 4Not in label — unassessed 19

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.
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. 13 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
Ischaemic stroke	42	49.6 36.9–66.8ROR 51.8 · χ² 1,950 / p <1e-300	48.5 EB05 37.4 · IC025 +4.5 3/3 methods agree	Not in label — unassessed	serious PRR 31.2 · ♀ 18% (drug 44%) · 90% clinician	no trial data cached
Parkinsonism	19	41.1 26.3–64.2ROR 41.9 · χ² 703 / p 3e-155	39.6 EB05 26.8 · IC025 +3.6 3/3 methods agree	Likely indication confounding term in indications; also in warnings	serious PRR 29.9	no trial data cached
Neuroleptic malignant syndrome	23	40.8 27.3–61.2ROR 41.8 · χ² 853 / p 7e-188	39.5 EB05 27.7 · IC025 +3.8 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 25.8 · 100% clinician	no trial data cached
Salivary hypersecretion	19	40.2 25.7–62.7ROR 40.9 · χ² 686 / p 2e-151	38.7 EB05 26.2 · IC025 +3.6 3/3 methods agree	Not in label — unassessed	serious PRR 34.3	no trial data cached
Respiratory distress	41	31.3 23.2–42.3ROR 32.6 · χ² 1,174 / p 2e-257	30.9 EB05 23.7 · IC025 +4.0 3/3 methods agree	Not in label — unassessed	serious PRR 20.0 · ♀ 3% (drug 44%) · 93% clinician	no trial data cached
Pneumonia aspiration	35	29.6 21.4–41.0ROR 30.7 · χ² 940 / p 1e-206	29.2 EB05 21.9 · IC025 +3.8 3/3 methods agree	Not in label — unassessed	serious PRR 18.7	no trial data cached
Extrapyramidal disorder	31	26.1 18.4–36.9ROR 26.9 · χ² 723 / p 2e-159	25.7 EB05 18.9 · IC025 +3.6 3/3 methods agree	Likely indication confounding term in indications; also in warnings	serious PRR 18.2 · 97% clinician	no trial data cached
Schizophrenia	15	26.0 15.7–43.0ROR 26.4 · χ² 336 / p 2e-75	25.2 EB05 16.1 · IC025 +3.0 3/3 methods agree	Not in label — unassessed	serious PRR 17.0	no trial data cached
Coma	50	22.1 16.9–29.0ROR 23.2 · χ² 989 / p 3e-217	21.9 EB05 17.3 · IC025 +3.7 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 14.0	no trial data cached
Altered state of consciousness	22	21.5 14.2–32.6ROR 22.0 · χ² 410 / p 1e-91	21.1 EB05 14.6 · IC025 +3.2 3/3 methods agree	Not in label — unassessed	serious PRR 13.7	no trial data cached
Muscle rigidity	11	19.4 10.8–34.9ROR 19.6 · χ² 174 / p 5e-40	18.3 EB05 10.1 · IC025 +2.4 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 13.7	no trial data cached
Delusion	14	19.1 11.3–32.1ROR 19.3 · χ² 222 / p 2e-50	18.3 EB05 11.2 · IC025 +2.7 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 12.4	no trial data cached
Dystonia	17	18.4 11.5–29.5ROR 18.7 · χ² 263 / p 2e-59	17.9 EB05 11.5 · IC025 +2.8 3/3 methods agree	Not in label — unassessed	serious PRR 12.1	no trial data cached
Acute respiratory distress syndrome	15	17.9 10.8–29.6ROR 18.1 · χ² 222 / p 1e-50	17.2 EB05 10.6 · IC025 +2.7 3/3 methods agree	Not in label — unassessed	serious PRR 11.3	no trial data cached
Poisoning	10	16.7 9.0–31.0ROR 16.9 · χ² 132 / p 6e-31	15.2 EB05 7.7 · IC025 +2.2 3/3 methods agree	Not in label — unassessed	serious PRR 10.7	no trial data cached
Product administration error	13	13.7 8.0–23.6ROR 13.9 · χ² 141 / p 8e-33	12.5 EB05 6.9 · IC025 +2.3 3/3 methods agree	Reporting / coding artifact	serious PRR 21.2	no trial data cached
Disease recurrence	31	13.7 9.7–19.4ROR 14.1 · χ² 353 / p 5e-79	13.3 EB05 9.6 · IC025 +2.9 3/3 methods agree	Reporting / coding artifact	serious PRR 17.1 · ♀ 0% (drug 44%) · 90% clinician	no trial data cached
Hallucination, auditory	10	13.2 7.1–24.6ROR 13.4 · χ² 101 / p 4e-24	11.3 EB05 5.7 · IC025 +2.0 3/3 methods agree	Not in label — unassessed	serious PRR 9.6	no trial data cached
Unresponsive to stimuli	16	13.1 8.1–21.3ROR 13.3 · χ² 167 / p 1e-38	12.1 EB05 7.3 · IC025 +2.4 3/3 methods agree	Not in label — unassessed	serious PRR 8.4	no trial data cached
Intestinal obstruction	21	11.8 7.7–18.0ROR 12.1 · χ² 198 / p 4e-45	11.0 EB05 7.2 · IC025 +2.5 3/3 methods agree	Not in label — unassessed	serious PRR 7.5	no trial data cached
Haematemesis (vomiting blood)	14	11.6 6.9–19.5ROR 11.8 · χ² 125 / p 2e-29	10.2 EB05 5.9 · IC025 +2.2 3/3 methods agree	Not in label — unassessed	serious PRR 7.5	no trial data cached
Urinary retention	18	11.4 7.2–18.0ROR 11.6 · χ² 161 / p 4e-37	10.3 EB05 6.5 · IC025 +2.4 3/3 methods agree	Labeled (related wording) related wording	serious PRR 8.1	no trial data cached
Peritonitis	11	11.2 6.2–20.2ROR 11.3 · χ² 92 / p 4e-22	9.3 EB05 5.0 · IC025 +1.9 3/3 methods agree	Not in label — unassessed	serious PRR 7.1	no trial data cached
Hallucination, visual	11	11.1 6.1–19.9ROR 11.2 · χ² 91 / p 8e-22	9.2 EB05 4.9 · IC025 +1.9 3/3 methods agree	Not in label — unassessed	serious PRR 8.0	no trial data cached
Dyskinesia (involuntary movements)	21	10.9 7.1–16.7ROR 11.1 · χ² 180 / p 3e-41	10.0 EB05 6.5 · IC025 +2.4 3/3 methods agree	Not in label — unassessed	serious PRR 9.4	no trial data cached
Rhabdomyolysis (muscle breakdown)	20	10.1 6.6–15.6ROR 10.3 · χ² 156 / p 5e-36	9.1 EB05 5.9 · IC025 +2.3 3/3 methods agree	Not in label — unassessed	serious PRR 6.4	no trial data cached
Hallucination (hallucinations)	36	10.1 7.3–14.0ROR 10.5 · χ² 288 / p 7e-65	9.6 EB05 7.1 · IC025 +2.6 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 7.7	no trial data cached
Tardive dyskinesia (persistent involuntary movements)	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 7.2	no trial data cached
Drug abuse	32	8.1 5.7–11.3ROR 8.3 · χ² 192 / p 7e-44	7.4 EB05 5.4 · IC025 +2.3 3/3 methods agree	Reporting / coding artifact	serious PRR 5.1 · 100% clinician	no trial data cached
Mental disorder	16	7.7 4.8–12.6ROR 7.8 · χ² 87 / p 4e-21	6.6 EB05 4.2 · IC025 +1.8 3/3 methods agree	Labeled (related wording) related wording	serious PRR 5.9	no trial data cached
Gastric disorder	10	6.1 3.3–11.2ROR 6.1 · χ² 37 / p 5e-10	4.9 EB05 2.8 · IC025 +1.2 3/3 methods agree	Not in label — unassessed	serious PRR 8.2	no trial data cached
Intentional overdose	15	5.0 3.0–8.2ROR 5.0 · χ² 44 / p 2e-11	4.4 EB05 2.8 · IC025 +1.3 3/3 methods agree	Reporting / coding artifact	serious PRR 3.2	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
Coma	50	22.1 16.9–29.0ROR 23.2 · χ² 989 / p 3e-217	21.9 EB05 17.3 · IC025 +3.7 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 14.0	no trial data cached
Ischaemic stroke	42	49.6 36.9–66.8ROR 51.8 · χ² 1,950 / p <1e-300	48.5 EB05 37.4 · IC025 +4.5 3/3 methods agree	Not in label — unassessed	serious PRR 31.2 · ♀ 18% (drug 44%) · 90% clinician	no trial data cached
Respiratory distress	41	31.3 23.2–42.3ROR 32.6 · χ² 1,174 / p 2e-257	30.9 EB05 23.7 · IC025 +4.0 3/3 methods agree	Not in label — unassessed	serious PRR 20.0 · ♀ 3% (drug 44%) · 93% clinician	no trial data cached
Hallucination (hallucinations)	36	10.1 7.3–14.0ROR 10.5 · χ² 288 / p 7e-65	9.6 EB05 7.1 · IC025 +2.6 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 7.7	no trial data cached
Pneumonia aspiration	35	29.6 21.4–41.0ROR 30.7 · χ² 940 / p 1e-206	29.2 EB05 21.9 · IC025 +3.8 3/3 methods agree	Not in label — unassessed	serious PRR 18.7	no trial data cached
Drug abuse	32	8.1 5.7–11.3ROR 8.3 · χ² 192 / p 7e-44	7.4 EB05 5.4 · IC025 +2.3 3/3 methods agree	Reporting / coding artifact	serious PRR 5.1 · 100% clinician	no trial data cached
Disease recurrence	31	13.7 9.7–19.4ROR 14.1 · χ² 353 / p 5e-79	13.3 EB05 9.6 · IC025 +2.9 3/3 methods agree	Reporting / coding artifact	serious PRR 17.1 · ♀ 0% (drug 44%) · 90% clinician	no trial data cached
Extrapyramidal disorder	31	26.1 18.4–36.9ROR 26.9 · χ² 723 / p 2e-159	25.7 EB05 18.9 · IC025 +3.6 3/3 methods agree	Likely indication confounding term in indications; also in warnings	serious PRR 18.2 · 97% clinician	no trial data cached
Neuroleptic malignant syndrome	23	40.8 27.3–61.2ROR 41.8 · χ² 853 / p 7e-188	39.5 EB05 27.7 · IC025 +3.8 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 25.8 · 100% clinician	no trial data cached
Altered state of consciousness	22	21.5 14.2–32.6ROR 22.0 · χ² 410 / p 1e-91	21.1 EB05 14.6 · IC025 +3.2 3/3 methods agree	Not in label — unassessed	serious PRR 13.7	no trial data cached
Dyskinesia (involuntary movements)	21	10.9 7.1–16.7ROR 11.1 · χ² 180 / p 3e-41	10.0 EB05 6.5 · IC025 +2.4 3/3 methods agree	Not in label — unassessed	serious PRR 9.4	no trial data cached
Intestinal obstruction	21	11.8 7.7–18.0ROR 12.1 · χ² 198 / p 4e-45	11.0 EB05 7.2 · IC025 +2.5 3/3 methods agree	Not in label — unassessed	serious PRR 7.5	no trial data cached
Rhabdomyolysis (muscle breakdown)	20	10.1 6.6–15.6ROR 10.3 · χ² 156 / p 5e-36	9.1 EB05 5.9 · IC025 +2.3 3/3 methods agree	Not in label — unassessed	serious PRR 6.4	no trial data cached
Salivary hypersecretion	19	40.2 25.7–62.7ROR 40.9 · χ² 686 / p 2e-151	38.7 EB05 26.2 · IC025 +3.6 3/3 methods agree	Not in label — unassessed	serious PRR 34.3	no trial data cached
Parkinsonism	19	41.1 26.3–64.2ROR 41.9 · χ² 703 / p 3e-155	39.6 EB05 26.8 · IC025 +3.6 3/3 methods agree	Likely indication confounding term in indications; also in warnings	serious PRR 29.9	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.

Ischaemic stroke — n=42, PRR 49.6
Respiratory distress — n=41, PRR 31.3
Pneumonia aspiration — n=35, PRR 29.6
Altered state of consciousness — n=22, PRR 21.5
Intestinal obstruction — n=21, PRR 11.8
Dyskinesia — n=21, PRR 10.9
Rhabdomyolysis — n=20, PRR 10.1

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

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 Trihexyphenidyl?

In FDA FAERS, the most frequently co-reported reaction terms for Trihexyphenidyl are coma, ischaemic stroke, respiratory distress, hallucination, pneumonia aspiration, extrapyramidal disorder. 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 Trihexyphenidyl have an FDA boxed warning?

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

Do FAERS signals prove that Trihexyphenidyl 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 294cd88d-d9e8-4938-b4be-c3adc4a35de0, effective 20220621; matched on openfda.generic_name.exact:"TRIHEXYPHENIDYL HYDROCHLORIDE")
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.