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Trihexyphenidyl is used to treat symptoms of Parkinson's disease or involuntary movements due to the side effects of certain psychiatric drugs (antipsychotics such as chlorpromazine/haloperidol). Trihexyphenidyl belongs to a class of medication called anticholinergics that work by blocking a certain natural substance (acetylcholine). This helps decrease muscle stiffness, sweating, and the production of saliva, and helps improve walking ability in people with Parkinson's disease. Anticholinergics can stop severe muscle spasms of the back, neck, and eyes that are sometimes caused by psychiatric drugs. It can also decrease other side effects such as muscle stiffness/rigidity (extrapyramidal signs-EPS). It is not helpful in treating movement problems caused by tardive dyskinesia and may worsen them.

If you miss a dose, take it as soon as you remember. If it is near the time of the next dose, skip the missed dose and resume your usual dosing schedule. Do not double the dose to catch up.

Take this medication by mouth, usually 3 to 4 times a day with meals and at bedtime, or as directed by your doctor. Your doctor may start you at a low dose and increase your dose slowly to find the best dose for you. The dosage is based on your medical condition and response to therapy. If you are using the liquid form of this medication, measure your dose with a special measuring spoon or device. Do not use a household spoon because it may not provide the correct dose. Take this medication regularly in order to get the most benefit from it. To help you remember, take it at the same times each day. Take this medication at least 1 hour before antacids containing magnesium, aluminum, or calcium. Allow at least 1-2 hours between doses of trihexyphenidyl and certain drugs for diarrhea (adsorbent antidiarrheals such as kaolin, pectin, attapulgite). Take this medication at least 2 hours after ketoconazole. Antacids and some drugs for diarrhea may prevent the full absorption of trihexyphenidyl, and this product may prevent the complete absorption of ketoconazole when these products are taken together. If you are taking this medication for side effects from another medication, your doctor may instruct you to take it on a regular schedule or only as needed. If you are taking this medication for Parkinson's disease, your doctor may change the dose of your other medications (e.g., levodopa). Follow your doctor's instructions closely. Rarely, abnormal drug-seeking behavior (addiction) is possible with this medication. Do not increase your dose, take it more frequently, or take it for a longer time than prescribed. Properly stop the medication when so directed. Some conditions may become worse when the drug is abruptly stopped. Your dose may need to be gradually decreased. When used for an extended period, this medication may not work as well and may require different dosing. Talk with your doctor if this medication stops working well. Inform your doctor if your condition persists or worsens.

Your healthcare professionals (e.g., doctor or pharmacist) may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop or change the dosage of any medicine before checking with them first. This drug should not be used with the following medication because very serious interactions may occur: pramlintide. If you are currently using the medication listed above, tell your doctor or pharmacist before starting trihexyphenidyl. Before using this medication, tell your doctor or pharmacist of all prescription and nonprescription/herbal products you may use, especially: amantadine, anticholinergics/antispasmodics (e.g., belladonna alkaloids, clidinium), certain antiarrhythmics (e.g., disopyramide, procainamide, quinidine), cholinesterase inhibitors (e.g., donepezil, galantamine), corticosteroids (e.g., prednisone), monoamine oxidase inhibitors (e.g., furazolidone, isocarboxazid, linezolid, moclobemide, phenelzine, procarbazine, selegiline, tranylcypromine), motion sickness medication (e.g., meclizine, scopolamine), narcotic pain relievers (e.g., meperidine), potassium tablets/capsules, tricyclic antidepressants (e.g., amitriptyline, doxepin). Tell your doctor or pharmacist if you also take drugs that cause drowsiness such as: certain antihistamines (e.g., diphenhydramine), anti-seizure drugs (e.g., carbamazepine), medicine for sleep or anxiety (e.g., alprazolam, diazepam, zolpidem), muscle relaxants, narcotic pain relievers (e.g., codeine), psychiatric medicines (e.g., risperidone, trazodone). Check the labels on all your medicines (e.g., cough-and-cold products) because they may contain anticholinergics or drowsiness-causing ingredients. Ask your pharmacist about the safe use of those products. This document does not contain all possible interactions. Therefore, before using this product, tell your doctor or pharmacist of all the products you use. Keep a list of all your medications with you, and share the list with your doctor and pharmacist.

Do not share this medication with others. Laboratory and/or medical tests (e.g., eye exams) should be performed periodically to monitor your progress or check for side effects. Consult your doctor for more details. People with Parkinson's disease may have an increased risk for developing skin cancer (melanoma). If you are taking this drug to treat Parkinson's disease, tell your doctor promptly if you notice a change in the appearance or size of moles or other unusual skin changes. Ask your doctor if you should have regular skin exams.

If overdose is suspected, contact your local poison control center or emergency room immediately. US residents can call the US national poison hotline at 1-800-222-1222. Canadian residents should call their local poison control center directly. Symptoms of overdose may include: unusually fast/slow heartbeat, slow/shallow breathing, unconsciousness, seizures, loss of coordination, fever, hot/dry/flushed skin, widened pupils, change in vision, change in the amount of urine, confusion, hallucinations.

Before taking trihexyphenidyl, tell your doctor or pharmacist if you are allergic to it; or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details. This medication should not be used if you have certain medical conditions. Before using this medicine, consult your doctor or pharmacist if you have: certain eye problem (narrow-angle glaucoma), blockage of the bladder/esophagus/stomach/intestines (e.g., bowel obstruction), severe ulcerative colitis. Before using this medication, tell your doctor or pharmacist your medical history, especially of: alcohol use, breathing problems (e.g., asthma, emphysema), diarrhea caused by an infection, certain eye problem (open-angle glaucoma), heart problems (e.g., angina, heart attack, heart failure, fast/irregular heartbeat), high/low blood pressure, intestinal problems (e.g., chronic constipation, ileus, ulcerative colitis), kidney disease, liver disease, mental/mood problems (e.g., anxiety, dementia, psychosis), certain muscle disease (myasthenia gravis), certain nerve disease (autonomic neuropathy), seizure, stomach problems (e.g., acid reflux, hiatal hernia, ulcer), stroke, overactive thyroid (hyperthyroidism), problems urinating (e.g., due to enlarged prostate, neurogenic bladder). This drug may make you dizzy or drowsy or cause blurred vision. Do not drive, use machinery, or do any activity that requires alertness or clear vision until you are sure you can perform such activities safely. Avoid alcoholic beverages. To minimize dizziness and lightheadedness, get up slowly when rising from a sitting or lying position. This medication decreases saliva production, an effect that can increase gum and tooth problems (e.g., cavities, gum disease). Take special care with your dental hygiene (e.g., brushing, flossing) and have regular dental check-ups. Liquid forms of this product may contain alcohol. Caution is advised if you have diabetes, alcohol dependence, or liver disease. Some medications (e.g., disulfiram, metronidazole) may cause a serious reaction when combined with alcohol. Tell your doctor if you are taking any medication that should not be used with alcohol. Ask your doctor or pharmacist about using this product safely. This drug can cause decreased sweating, which could cause a severe rise in your body temperature (hyperthermia). The risk of this serious side effect is greater in hot weather, during vigorous exercise, and/or if you drink alcohol. Drink plenty of fluids and dress lightly while in hot weather and when exercising. If you experience signs of hyperthermia such as mental/mood changes, headache, or dizziness, promptly seek cool or air-conditioned shelter and/or stop exercising, and seek immediate medical attention. Consult your doctor for more details. The elderly may be more sensitive to the effects of this drug, especially dizziness, heatstroke, memory problems, and constipation. Children may be more sensitive to the effects of this drug, especially effects on heart rate. This drug should be used only when clearly needed during pregnancy. Discuss the risks and benefits with your doctor. It is not known if this drug passes into breast milk. Consult your doctor before breast-feeding.

Store at room temperature between 68-77 degrees F (20-25 degrees C) away from light and moisture. Do not store in the bathroom. Keep all medicines away from children and pets. Do not flush medications down the toilet or pour them into a drain unless instructed to do so. Properly discard this product when it is expired or no longer needed. Consult your pharmacist or local waste disposal company for more details about how to safely discard your product.

Drowsiness, dizziness, constipation, flushing, nausea, nervousness, blurred vision, or dry mouth may occur. These effects usually lessen as your body gets used to the medicine. If any of these effects persist or worsen, contact your doctor or pharmacist promptly. To relieve dry mouth, suck on (sugarless) hard candy or ice chips, chew (sugarless) gum, drink water, or use a saliva substitute. Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects. Tell your doctor immediately if any of these unlikely but serious side effects occur: decreased sexual ability, severe stomach/abdominal pain, difficult/painful swallowing, difficulty urinating, vision changes, weakness. Seek immediate medical attention if any of these very unlikely but very serious side effects occur: chest pain, severe dizziness/fainting, high fever, fast/irregular/slow heartbeat, mental/mood changes (e.g., confusion, hallucinations, memory problems). A serious allergic reaction to this drug is unlikely, but seek immediate medical attention if it occurs. Symptoms of a serious allergic reaction include: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.