Trihexyphenidyl (Oral)

As an anticholinergic, Trihexyphenidyl blocks acetylcholine to decrease feelings of stiffness in the muscles, sweating, and the production of saliva.


Trihexyphenidyl helps Parkinson's patients to walk more easily and reduces back and neck muscle spasms, which are typically caused by drugs used to treat psychosis. While Trihexyphenidyl can be used to alleviate muscle stiffness and rigidity, it's not recommended to treat tardive dyskinesia. In some cases, Trihexyphenidyl has actually caused tardive dyskinesia to become worse.

In addition to counteracting the side effects of antipsychotic drugs, Trihexyphenidyl can also be used to alleviate adverse reactions created by drugs to treat other conditions. Medications used to treat nausea, anxiety, depression, and other emotional conditions can often have unexpected side effects related to muscle control. Trihexyphenidyl can help alleviate those effects and make it easier for patients to function in their day to day lives.

Conditions treated

  • Parkinson's disease
  • Nausea
  • Psychological/emotional conditions
  • Nervousness
  • Counteract antipsychotic drugs

Type of Medicine

  • Anticholinergic

Side Effects

The most common side effects experienced by patients taking Trihexyphenidyl have been drowsiness and lightheadedness. Nausea, constipation, and increased feelings of nervousness have also been reported. Patients also reported blurred vision and an excessive dryness in the mouth and throat, while taking Trihexyphenidyl.

As the patient continues to take the drug as prescribed, the aforementioned symptoms should lessen. If they persist over time or the side effects worsen, the patient is urged to follow up with his or her doctor immediately. It's recommended that a dry mouth can be treated by sucking on sugar-free hard candy, ice chips, or by chewing sugarless gum or drinking cold water.

Typically, most patients only experience minimal side effects while on Trihexyphenidyl, if they observe any adverse effects at all. The drug is intended to alleviate the previously mentioned symptoms and, as such, is generally prescribed because the benefit is greater than the risk of experiencing side effects.

Other side effects experienced less frequently, but often enough to warrant mentioning, include a decreased libido, stomach pain and/or spasms, trouble swallowing, difficulty in urinating, and a general feeling of weakness. Far less likely, but possible, side effects have included chest pain, more severe dizziness that may include fainting, and a high fever. Also, patients taking Trihexyphenidyl should be wary of experiencing irregular (faster or slower) heartbeat and eye swelling or redness, which may also include pain. If the patient experiences any of the aforementioned conditions, he or she should seek immediate medical attention.

Another rare side effect of taking Trihexyphenidyl concerns the way the drug affects the psychology of the patient. He or she may experience extreme and frequent mood swings, confusion, hallucinations, and memory problems. There may also be allergic reactions to Trihexyphenidyl with some patients, though incidences of this have been extremely rare. Symptoms of an allergic reaction are a rash; swelling and itching of the face, tongue, and throat; extreme dizziness or lightheadedness; and difficulty breathing. Here, also, medical attention should be immediately sought, if these symptoms are experienced.


As with all medications, it's very important to take Trihexyphenidyl only as prescribed by the physician. This means patients should avoid taking more of the drug than advised, either in frequency or in the size of the dose. Additionally, patients should stop taking the medication when advised to do so by their doctor, even if they still have a supply remaining.

Trihexyphenidyl must be taken with food, though the patient may take it before or after the meal, as well. As the dosage will vary according to each patient's needs, it's important to pay close attention to the instructions of the doctor at the time he or she prescribes the medication. Otherwise, the instructions printed on the side of the bottle may also be followed, as they should be similar. In determining the patient's dosage, the physician takes a number of different factors into account. The strength of the medicine, number of doses per day, the time between doses, and the period of time the patient is to be taking the medicine all play a part in determining dose size. It can also depend on the reason for which the patient was prescribed Trihexyphenidyl.

The manufacturers of Trihexyphenidyl have provided general dosage instructions, but it should be reiterated that these recommendations may be altered by the patient's doctor. Factors discussed between physician and patient will be the main concern in establishing dose size and frequency.

For treating symptoms related to Parkinson's disease, the recommended dosage for adults is 1 milligram (mg) just once per day to start. Once the patient's doctor has time to see the effect, he may increase the dosage. Typically, Trihexyphenidyl isn't taken in excess of 5 mg per day, which is usually divided up between three or four times each day.

In taking Trihexyphenidyl as a means of reducing the side effects of other medications, adults should take between 5 and 15 milligrams per day. Again, those doses should be divided up and taken three or four times each day.

There are no recommended dosage sizes for children. Instead, the drug manufacturer leaves the dosage size and frequency up to the administering physician.

Patients are warned against taking double doses. If he or she misses a dose, they can take the missed dose upon realizing it. If it's close to the time at which the patient takes the next dose, he or she should omit the missed dose and just take the planned next dose.

If the patient experiences signs of an overdose (extremely slow or fast heartbeat, shallow or difficult breathing, unconsciousness, seizures, loss of coordination, or flushed skin), they need immediate attention. Either call 911 or the Poison Control Center (1-800-222-1222) immediately.

Major Drug Interactions

Drugs interact with other drugs in the human body, changing the effects of each medication. This may cause one medication to be ineffective in treating a condition or it may cause a dangerous reaction in the patient. For these reasons, it's important for patients to keep a list of all medications they are currently taking, including the dosage and frequency for each drug. Where some patients fail is in assuming that this applies only to prescribed medication. To the contrary, it's important that your doctor knows everything you're taking, including non-prescription, over the counter drugs.

Below is a partial list of the major drugs known to have negatively interacted with Trihexyphenidyl. If the patient is already taking any of these medications, he or she should notify their doctor before taking the first dose of Trihexyphenidyl:

  • Amantadine
  • Anticholinergics/antispasmodics
  • Antiarrhythmics, especially disopyramide and quinidine
  • Corticosteroids
  • MAO inhibitors (These include isocarboxazid, linezolid, methylene blue, moclobemide, phenelzine, procarbazine, rasagiline, safinamide, selegiline, and tranylcypromine.)
  • Meclizine and scopolamine, as well as other motion sickness medications
  • Potassium tablets/capsules
  • Tricyclic antidepressants, such as amitriptyline and doxepin

Additionally, patients are urged to tell their doctors if they are taking anything else that might induce drowsiness. This includes alcohol and marijuana, as well as sleeping pills and cough suppressants that may contain codeine and hydrocodone. The patient's doctor should also be made aware of any drug used to control anxiety and any form of muscle relaxants that the patient may be taking. The patient should be sure to check the ingredient listing on each medication he or she may be taking. Some drugs do contain ingredients intended to cause drowsiness.


In addition to advising the doctor of allergies and current drugs being taken, the patient should also discuss his or her medical history and current health problems. Trihexyphenidyl should not be taken: where the patient suffers from com/health/coma/">glaucoma (currently or in the past); a bladder, esophagus, stomach, or intestinal blockage; or severe ulcerative colitis. Additionally, the patient should notify his or her doctor of a history of any of the following conditions, as they may affect the patient's ability to take Trihexyphenidyl:

  • Asthma, emphysema, or any other breathing problems
  • Diarrhea which has been induced by an infection
  • Poor heart health, including angina, heart attack, heart failure,or an irregular heartbeat
  • High or low blood pressure
  • Intestinal problems
  • Kidney disease
  • Liver disease
  • Muscle disease
  • Nerve disease
  • Urination problems
  • Mood or emotional problems

Additionally, the doctor should be made aware of any addictions the patient has experienced. This includes past addictions for which the patient may have successfully completed a rehabilitation or treatment program. As this drug can make the patient feel lightheaded, dizzy, and drowsy, he or she should also be advised against driving an automobile or operating heavy machinery. In fact, the patient shouldn't engage in any activity that requires an alert mind or clear vision, until he or she feels certain that the tasks can be performed safely.

Patients should also avoid drinking alcohol while taking Trihexyphenidyl. This is especially true in cases where the drug is prescribed in liquid form, as it may already carry small amounts of alcohol. The effects of Trihexyphenidyl tend to simulate intoxication, as far as lightheadedness and dizziness, so patients are urged to sit up and move about with caution.


Trihexyphenidyl should be stored at room temperature. Freezing temperatures should be avoided at all times. Additionally, the drug's manufacturer recommends that it be kept away from heat, moisture, and direct light. Ideally, it should be kept in a cupboard or medicine cabinet, where children cannot reach it.

Once the doctor and patient determine Trihexyphenidyl is no longer necessary for treatment, they should discuss proper disposal of the remaining supply. Medication that is either expired or no longer used should be disposed of without haste. Healthcare professionals can advise patients on how best to get rid of the remaining supply.


While Trihexyphenidyl is a greatly beneficial drug, it can also pose a risk to patients who fail to communicate fully with their physicians. As a treatment designed to alleviate the symptoms of Parkinson's disease and the effects of other medications, Trihexyphenidyl reduces muscle tension and treats the overproduction of saliva and perspiration, but it also affects how the body reacts to stimuli. By producing feelings of drowsiness and dizziness, the drug can impair the patient's day to day functioning and create perilous situations, if proper precautions aren't taken. For this reason, it's important that the patient tells his or her doctor as much as possible about their own medical history, as well as family histories that may be relevant. There are hundreds of medications that can cause adverse reactions with Trihexyphenidyl and dozens of medical conditions that could be worsened by use of the drug. This makes it necessary for the patient's own best interests to be honest and forthcoming with his or her doctor.

When taken correctly, Trihexyphenidyl can provide the relief of side effects and symptoms, which may have been previously untreatable. Whether the patient suffers from Parkinson's or experiences extreme side effects from another medication, Trihexyphenidyl can alleviate those symptoms. In some cases, that means allowing a patient to walk with more confidence, reducing staggering and muscle spasms, or it may mean giving a patient greater freedom of movement. To achieve these results, patient and doctor must work together to find the appropriate dosage and the best frequency of use.

Last Reviewed:
December 22, 2017
Last Updated:
April 05, 2018
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