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How does this medication work? What will it do for me?

Trihexyphenidyl belongs to the family of medications called antiparkinsonians. It is used to treat symptoms of Parkinson's disease. It may also be used to reduce the shakiness and restlessness that can be caused by some tranquilizers. It acts on the nervous system and corrects some of the chemical imbalances that cause Parkinson's disease as well as the symptoms caused by some medications.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

How should I use this medication?

For treatment of Parkinson's disease, the recommended dose of trihexyphenidyl starts with 1 mg the first day and then increases by 2 mg daily at intervals of 3 to 5 days until symptoms are controlled. The usual daily dose ranges from 6 mg to 10 mg daily depending on needs (taken in divided doses at mealtimes).

The dose needed for medication-induced symptoms varies widely according to the particular medication and dose that are causing the symptoms.

Many things can affect the dose of a medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

It is important to take this medication exactly as prescribed by your doctor. If you miss a dose, take it as soon as possible and continue with your regular schedule. If it is almost time for your next dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Store this medication at room temperature, protect it from light and moisture, and keep it out of the reach of children.

What form(s) does this medication come in?

pms-Trihexyphenidyl is no longer being manufactured for sale in Canada. For brands that may still be available, search under trihexyphenidyl. This article is being kept available for reference purposes only. If you are using this medication, speak with your doctor or pharmacist for information about your treatment options.

Who should NOT take this medication?

Do not take trihexyphenidyl if you:

  • are allergic to trihexyphenidyl or to any of the ingredients of the medication
  • have angle-closure glaucoma
  • have severe ulcerative colitis
What side effects are possible with this medication?

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent. The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

  • blurred vision
  • constipation
  • decreased sweating
  • difficult or painful urination (especially for senior men)
  • dizziness or lightheadedness when rising from a lying or sitting position
  • drowsiness
  • dryness of mouth, nose, or throat
  • false sense of well-being (especially for seniors or those taking high doses)
  • headache
  • increased sensitivity of eyes to light
  • muscle weakness
  • nausea or vomiting
  • nervousness
  • soreness of mouth and tongue
  • stomach upset or pain
  • unusual excitement

Although most of these side effects listed below don't happen very often, they could lead to serious problems if you do not check with your doctor or seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

  • confusion (more common for seniors or those taking high doses)
  • eye pain
  • fast or pounding heartbeat
  • hallucinations (seeing, hearing, or feeling things that are not there)
  • skin rash

Stop taking the medication and seek immediate medical attention if any of the following occur:

  • symptoms of a severe allergic reaction (e.g., hives, difficulty breathing, swelling of the throat or mouth)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Are there any other precautions or warnings for this medication?

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Drowsiness/reduced alertness: Trihexyphenidyl may impair the mental or physical abilities required to perform hazardous tasks such as driving or operating machinery. Avoid these activities if the medication affects you in this way.

Hot conditions: Trihexyphenidyl may reduce the amount of perspiration produced. This can affect the body's ability to cool itself. Try to avoid overheating due to hot weather and heavy exercise.

Medical conditions: People with the following medical conditions should discuss with their doctor how this medication may affect their medical condition, how their medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed:

  • chronic obstructive pulmonary disease
  • gastroesophageal reflux disease (GERD)
  • heart disease
  • high blood pressure
  • kidney disease
  • liver disease
  • prostate problems
  • trouble with stomach emptying
  • stomach infections
  • stomach ulcers
  • ulcerative colitis

Pregnancy: The safety and effectiveness of using this medication have not been established for pregnant women. If you become pregnant while taking this medication, contact your doctor immediately.

Breast-feeding: It is not known if trihexyphenidyl passes into breast milk. If you are breast-feeding and are taking this medication, it may affect your baby. Talk to your doctor about whether you should continue breast-feeding.

Children: The safety and effectiveness of using this medication have not been established for children.

Seniors: Seniors may be at a higher risk for side effects and may require lower doses.

What other drugs could interact with this medication?

There may be an interaction between trihexyphenidyl and any of the following:

  • aclidinium
  • alcohol
  • amantadine
  • amoxapine
  • antihistamines (e.g., bilastine, chlorpheniramine, diphenhydramine, rupatadine)
  • antipsychotics (e.g., clozapine, loxapine, haloperidol, olanzapine, pimozide, quetiapine)
  • benztropine
  • bilastine
  • botulinum toxin
  • cannabis
  • clidinium
  • clozapine
  • cholinesterase inhibitors (e.g., donepezil, galantamine, rivastigmine)
  • darifenacin
  • disopyramide
  • divalproex
  • domperidone
  • eluxadoline
  • fesoterodine
  • glycopyrrolate
  • hydrochlorothiazide
  • indapamide
  • MAO inhibitors (e.g., moclobemide, tranylcypromine, phenelzine)
  • methadone
  • metoclopramide
  • metolazone
  • mirabegron
  • muscle relaxants (e.g., cyclobenzaprine, orphenadrine)
  • nabilone
  • narcotic pain medications (e.g., codeine, fentanyl, hydromorphone, morphine)
  • nitroglycerin
  • oxybutynin
  • potassium chloride
  • scopolamine
  • solifenacin
  • tiotropium
  • tolterodine
  • topiramate
  • tricyclic antidepressants (e.g., amitriptyline, desipramine)
  • trospium
  • umeclidinium
  • valproic acid

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

  • stop taking one of the medications,
  • change one of the medications to another,
  • change how you are taking one or both of the medications, or
  • leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.

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