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.

Take this medication by mouth as directed by your doctor, usually 3 to 4 times a day with meals and at bedtime. 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, age, and response to treatment.

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 (such as levodopa). Follow your doctor's instructions closely.

Though it helps many people, this medication may sometimes cause addiction. This risk may be higher if you have a substance use disorder (such as overuse of or addiction to drugs/alcohol). Do not increase your dose, take it more often, or use it for a longer time than prescribed. Properly stop the medication when so directed. Some conditions may become worse when the drug is suddenly 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. Tell your doctor if your condition lasts or gets worse.

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 last or get worse, 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.

To reduce the risk of dizziness and lightheadedness, get up slowly when rising from a sitting or lying position.

Remember that this medication has been prescribed because your doctor 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 right away if you have any serious side effects, including:

• decreased sexual ability
• severe stomach/abdominal pain
• difficult/painful swallowing
• difficulty urinating
• weakness

Get medical help right away if you have any very serious side effects, including:

• chest pain
• severe dizziness/fainting
• high fever
• fast/irregular/slow heartbeat
• mental/mood changes (such as confusion, hallucinations, memory problems)
• eye pain/swelling/redness
• vision changes (such as seeing rainbows around lights at night)

A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including:

• 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 or at www.fda.gov/medwatch.

In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

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.

Before using this medication, tell your doctor or pharmacist your medical history, especially of:

• glaucoma
• breathing problems (such as asthma, emphysema)
• diarrhea caused by an infection
• heart problems (such as angina, heart attack, heart failure, fast/irregular heartbeat)
• high/low blood pressure
• stomach/intestinal problems (such as chronic constipation, ileus, blockage, ulcerative colitis, acid reflux, ulcer)
• kidney disease
• liver disease
• mental/mood problems (such as anxiety, dementia, psychosis)
• certain muscle disease (myasthenia gravis)
• certain nerve disease (autonomic neuropathy)
• seizure
• stroke
• overactive thyroid (hyperthyroidism)
• problems urinating (such as due to enlarged prostate, neurogenic bladder)
• personal or family history of a substance use disorder (such as overuse of or addiction to drugs/alcohol)

This drug may make you dizzy or drowsy or blur your vision. Alcohol or marijuana (cannabis) can make you more dizzy or drowsy. Do not drive, use machinery, or do anything that needs alertness or clear vision until you can do it safely. Avoid alcoholic beverages. Talk to your doctor if you are using marijuana (cannabis).

This medication decreases saliva production, an effect that can increase gum and tooth problems (such as cavities, gum disease). Take special care with your dental hygiene (such as 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 (such as 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 get medical help right away. Consult your doctor for more details.

Older adults may be more sensitive to the side effects of this drug, especially dizziness, drowsiness, heatstroke, memory problems, difficulty urinating, and constipation. Dizziness and drowsiness can increase the risk of falling.

Children may be more sensitive to the side effects of this drug, especially the effects on heart rate.

During pregnancy, this medication should be used only when clearly needed. Discuss the risks and benefits with your doctor.

It is unknown if this drug passes into breast milk. Consult your doctor before breastfeeding.

Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor's approval.

Some products that may interact with this drug are:

• pramlintide
• corticosteroids (such as prednisone)
• potassium tablets/capsules

Tell your doctor or pharmacist if you are taking other products that cause drowsiness such as opioid pain or cough relievers (such as codeine, hydrocodone), alcohol, marijuana (cannabis), drugs for sleep or anxiety (such as alprazolam, lorazepam, zolpidem), muscle relaxants (such as carisoprodol, cyclobenzaprine), or antihistamines (such as cetirizine, diphenhydramine).

Check the labels on all your medicines (such as allergy or cough-and-cold products) because they may contain ingredients that cause drowsiness. Ask your pharmacist about using those products safely.

If someone has overdosed and has serious symptoms such as passing out or trouble breathing, call 911. Otherwise, call a poison control center right away. US residents can call 1-800-222-1222. Canada residents can call 1-844-764-7669. 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.

Do not share this medication with others.

Lab and/or medical tests (such as eye exams) should be done while you are taking this medication. Keep all medical and lab appointments. Consult your doctor for more details.

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. Take your next dose at the regular time. Do not double the dose to catch up.

Store at room temperature away from light and moisture. Do not store in the bathroom. Keep all medications 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.