TRIHEXYPHENIDYL HYDROCHLORIDE
Clinical safety rating: caution
Comprehensive clinical and safety monograph for TRIHEXYPHENIDYL HYDROCHLORIDE (TRIHEXYPHENIDYL HYDROCHLORIDE).
Trihexyphenidyl is an anticholinergic agent that competitively blocks central muscarinic receptors (primarily M1) in the striatum, restoring the balance between acetylcholine and dopamine in the basal ganglia. It also has mild peripheral anticholinergic effects.
| Metabolism | Primarily metabolized by hepatic microsomal enzymes, with CYP2D6 possibly involved. Elimination half-life is approximately 3–4 hours. |
| Excretion | Renal (primarily as unchanged drug and metabolites; <15% unchanged) and biliary/fecal (minor). |
| Half-life | 10-17 hours; clinical context: steady-state concentrations achieved in 2-3 days. |
| Protein binding | Minimal; approximately 10-20% bound to plasma proteins (albumin). |
| Volume of Distribution | Approximately 0.5-1.5 L/kg; indicates extensive tissue distribution. |
| Bioavailability | Oral: approximately 80-95% (first-pass metabolism minimal). |
| Onset of Action | Oral: 1-2 hours; IM: 10-15 minutes; IV: immediate. |
| Duration of Action | Oral: 6-12 hours; IM/IV: 2-4 hours. Note: effects persist longer with chronic use. |
| Molecular Weight | 337.93 |
1 mg orally initially, then increase by 2 mg every 3-5 days up to 6-10 mg daily in 3-4 divided doses; maximum 15 mg/day.
| Dosage form | TABLET |
| Renal impairment | CrCl 10-50 mL/min: administer every 8-12 hours; CrCl <10 mL/min: consider alternative or reduce dose by 50%. |
| Liver impairment | Child-Pugh Class A: no adjustment; Class B: reduce dose by 50%; Class C: avoid use or use with extreme caution, reduce dose by 75%. |
| Pediatric use | Children 2-12 years: initial 1 mg/day, increase by 1 mg every 3-5 days; maximum 6 mg/day in divided doses. |
| Geriatric use | Start at 1 mg once or twice daily; increase slowly by 1 mg increments every 5-7 days; monitor for anticholinergic side effects. |
| 1st trimester | Limited human data; animal studies show no teratogenicity at clinically relevant doses. Avoid unless benefits outweigh risks. |
| 2nd trimester | Use only if clearly needed; consider potential anticholinergic effects on fetus. |
| 3rd trimester | Risk of neonatal anticholinergic effects (e.g., ileus, urinary retention) if used near term. |
Clinical note
Comprehensive clinical and safety monograph for TRIHEXYPHENIDYL HYDROCHLORIDE (TRIHEXYPHENIDYL HYDROCHLORIDE).
| Placental transfer | Crosses placenta; extent unknown but likely significant due to molecular weight. |
| Breastfeeding | Excretion into breast milk is minimal; unlikely to cause adverse effects in infant. Monitor for anticholinergic symptoms. |
| Lactation Rating |
■ FDA Black Box Warning
Not applicable.
| Serious Effects |
GlaucomaPyloric or duodenal obstructionStenosing peptic ulcersProstatic hypertrophy with urine retentionMyasthenia gravis
| Precautions | May cause drowsiness, dizziness, or blurred vision; caution with driving or hazardous activities, Anticholinergic effects may be exacerbated in elderly patients, including confusion, constipation, urinary retention, and hyperthermia, Use cautiously in patients with glaucoma, prostatic hypertrophy, cardiac arrhythmias, or myasthenia gravis, Potential for abuse or dependence at high doses, May exacerbate tardive dyskinesia in patients on neuroleptics |
| Food/Dietary | No significant food interactions. However, avoid excessive alcohol consumption as it may exacerbate CNS depression and anticholinergic effects. Maintain adequate hydration to prevent constipation and dry mouth. |
Loading safety data…
| L2 (Probably Compatible) |
| Teratogenic Risk | First trimester: Limited human data; animal studies show no consistent teratogenicity. Second and third trimesters: No known specific risks; use only if clearly needed. |
| Fetal Monitoring | Monitor maternal extrapyramidal symptoms and anticholinergic adverse effects. Fetal monitoring not specifically required unless maternal toxicity occurs. |
| Fertility Effects | No known adverse effects on human fertility; animal studies show no impairment. |
| Clinical Pearls | Trihexyphenidyl is an anticholinergic agent used primarily for drug-induced parkinsonism and idiopathic Parkinson disease. It is less effective than levodopa but useful as adjunctive therapy. Onset of action is within 1 hour after oral administration; peak effect at 2-3 hours. Monitor for anticholinergic side effects: dry mouth, blurred vision, constipation, urinary retention, and cognitive impairment. Use with caution in elderly patients due to increased risk of confusion and falls. Abrupt withdrawal may precipitate parkinsonian crisis; taper gradually. Avoid in patients with narrow-angle glaucoma, myasthenia gravis, or gastrointestinal obstruction. |
| Patient Advice | Take exactly as prescribed; do not stop suddenly without consulting your doctor. · This medication may cause dry mouth, blurred vision, constipation, or difficulty urinating. · Avoid alcohol and other CNS depressants as they may increase drowsiness and dizziness. · Use caution when driving or operating machinery until you know how this drug affects you. · Report any eye pain, vision changes, or difficulty passing urine to your healthcare provider. · Do not chew sustained-release capsules; swallow whole. · If you miss a dose, take it as soon as you remember unless it is almost time for your next dose; do not double doses. |