Trihexyphenidyl is used to treat a group of side effects (called parkinsonian side effects) that include tremors, difficulty walking, and slack muscle tone. These side effects may occur in patients who are taking antipsychotic medications used to treat mental disorders such as schizophrenia.
Some medicines, called antipsychotic drugs, that are used to treat schizophrenia and other mental disorders can cause side effects that are similar to the symptoms of Parkinson's disease. The patient does not have Parkinson's disease, but he or she may experience shaking in muscles while at rest, difficulty with voluntary movements, and poor muscle tone. These symptoms are similar to the symptoms of Parkinson's disease.
One way to eliminate these undesirable side effects is to stop taking the antipsychotic medicine. Unfortunately, the symptoms of the original mental disorder usually come back, so in most cases simply stopping the antipsychotic medication is not a reasonable option. Some drugs such as trihexyphenidyl that control the symptoms of Parkinson's disease also control the parkinsonian side effects of antipsychotic medicines.
Trihexyphenidyl works by restoring the chemical balance between dopamine and acetylcholine, two neurotransmitter chemicals in the brain. Taking trihexyphenidyl along with the antipsychotic medicine helps to control symptoms of the mental disorder, while reducing parkinsonian side effects. Trihexyphenidyl is in the same family of drugs (commonly known as anticholinergic drugs) as biperidenand benztropine.
Trihexyphenidyl is available in 2-mg and 5-mg tablets and an elixir containing 2 mg per teaspoonful. For the treatment of tremor, poor muscle tone, and similar side effects, trihexyphenidyl should be started at a dose of 1 to 2 mg orally two to three times daily or as needed, to a maximum daily dose of 15 mg per day. Parkinson-like side effects caused by antipsychotic drugs may come
Trihexyphenidyl should never be used in children under age three. It should be used cautiously and with close physician supervision in older children and in people over age 60. Trihexyphenidyl, like all anticholinergic drugs, decreases sweating and the body's ability to cool itself. People who are unaccustomed to being outside in hot weather should take care to stay as cool as possible and drink extra fluids. People who are chronically ill, have a central nervous system disease, or who work outside during hot weather may need to avoid taking trihexyphenidyl.
People who have the following medical problems may experience increased negative side effects when taking trihexyphenidyl. Anyone with these problems should discuss their condition with their physician before starting the drug:
- glaucoma, especially closed-angle glaucoma
- intestinal obstruction
- prostate enlargement
- urinary bladder obstruction
Although rare, some patients experience euphoria while taking trihexyphenidyl and may abuse it for this reason. Euphoria can occur at doses only two to four times the normal daily dose. Patients with a history of drug abuse should be observed carefully for trihexyphenidyl abuse.
Although trihexyphenidyl helps to control the side effects of antipsychotic drugs, it can produce side effects of its own. A person taking trihexyphenidyl may have some of the following reactions, which may vary in intensity:
- dry mouth
- dry skin
- blurred vision
- nausea or vomiting
- increased heart rate
- urinary retention
Dry mouth, if severe to the point of causing difficulty speaking or swallowing, may be managed by reducing or temporarily discontinuing trihexyphenidyl. Chewing sugarless gum or sucking on sugarless candy may also help to increase the flow of saliva. Some artificial saliva products may give temporary relief.
Men with prostate enlargement may be especially prone to urinary retention. Symptoms of this problem include having difficulty starting a urine flow and more difficulty passing urine than usual. This side effect may be severe and require discontinuation of the drug. Urinary retention may require catheterization. People who think they may be experiencing any side effects from this or any other medication should tell their physicians.
Patients who take an overdose of trihexyphenidyl are treated with forced vomiting, removal of stomach contents and stomach washing, activated charcoal, and respiratory support if needed. They are also given physostigmine, an antidote for anticholinergic drug poisoning.
When drugs such as trihexyphenidyl are taken with antidepressants such as amitriptyline, imipramine, trimipramine, desipramine, nortriptyline, protriptyline, amoxapine, and doxepinor with many antihistamines that also have anticholinergic properties, the effects and side effects of trihexyphenidyl are usually intensified.
Drugs such as trihexyphenidyl decrease the speed with which food moves through the stomach and intestines. Because of this, the absorption of other drugs being taken may be enhanced by trihexyphenidyl. Patients receiving trihexyphenidyl should be alert to unusual responses to other drugs they might be taking and report any changes to their physician.
American Society of Health-System Pharmacists. AHFS Drug Information 2002.Bethesda: American Society of Health-System Pharmacists, 2002.
DeVane, C. Lindsay, Pharm.D. "Drug Therapy for Psychoses." In Fundamentals of Monitoring Psychoactive Drug Therapy.Baltimore: Williams and Wilkins, 1990.
Jack Raber, Pharm.D.