Trihexyphenidyl (THP,  benzhexol, trihex, marketed as Artane and others) is an antispasmodic drug used to treat stiffness, tremors, spasms, and poor muscle control. It is an agent of the antimuscarinic class and is often used  in management of Parkinsons disease. It was approved by the FDA for the treatment of Parkinsons in the US in 2003.It is on the World Health Organizations List of Essential Medicines. Medical uses
Trihexyphenidyl is used for the symptomatic treatment of Parkinsons disease in mono and combination therapy. It is active in postencephalitic, arteriosclerotic, and idiopathic forms. The drug is also commonly used to treat extrapyramidal side effects occurring during antipsychotic treatment. It reduces the frequency and duration of oculogyric crises as well as of dyskinetic movements and spastic contractions. Trihexyphenidyl may improve psychotic depression and mental inertia frequently associated with Parkinsons disease and symptomatic problems caused by antipsychotic treatment.The drug cannot cure Parkinsons disease, but may provide substantial alleviation of symptoms. An estimated 50–75% of people with Parkinsons disease will react positively and experience a 20–30% symptomatic improvement. To increase therapeutic activity, trihexyphenidyl is often given concomitantly with levodopa or other antimuscarinic or antihistaminic (e.g. diphenhydramine) agents. Combination therapy with dopamine agonists such as cabergoline is also possible. This is often termed a multidimensional approach. It has also been prescribed for essential tremor and akathisia.
Contraindications
Contradindications include:
Hypersensitivity to trihexyphenidyl
Narrow angle glaucoma
Ileus (disruption of the normal propulsive ability of the intestine)
Caution: People with obstructive diseases of the urogenital tract, people with a known history of seizures and those with potentially dangerous tachycardia
People under 18 years of age should not be treated due to a lack of clinical experience. People should allow a period to adjust to the dose when first starting trihexyphenidyl and when the dose has been increased or added to a regimen with other drugs because acute somnolence and accumulated fatigue can make it particularly dangerous to operate an automobile, heavy machinery etc. Adverse effects
Dose-dependent side effects are frequent, but typically lessen over time as the body adapts to the medication. All of the following symptoms considered, Artane has been shown to dramatically and consistently improve neurologic defects in people aged 16–86 over the course of five years. People who are older or who have psychiatric conditions may become confused or develop delirium. Side effects include but are not limited to:
Central nervous system: drowsiness, vertigo, headache, and dizziness are frequent. With high doses nervousness, agitation, anxiety, delirium, and confusion are noted. Trihexyphenidyl may be abused due to a short acting mood-elevating and euphoric effect. The normal sleep architecture may be altered (REM sleep depression). Trihexyphenidyl may lower the seizure-threshold. Peripheral side effects: dry mouth, impaired sweating, abdominal discomfort, nausea, and constipation are frequent. Tachycardia or heart palpitations (fast heart rate) may be noted. Allergic reactions are rare, but may occur.