SIDE EFFECTS: 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 right away if any of these unlikely but serious side effects occur: 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 (e.g., confusion, hallucinations, memory problems), eye pain/swelling/redness, vision changes (such as seeing rainbows around lights at night). 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 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.

PRECAUTIONS: 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: personal or family history of glaucoma (angle-closure type), 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: breathing problems (e.g., asthma, emphysema), diarrhea caused by an infection, 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), 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 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. 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. 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 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.

DRUG INTERACTIONS: 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. 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: anticholinergics/antispasmodics (e.g., belladonna alkaloids, clidinium), certain antiarrhythmics (e.g., disopyramide, quinidine), corticosteroids (e.g., prednisone), MAO inhibitors (isocarboxazid, linezolid, methylene blue, moclobemide, phenelzine, procarbazine, rasagiline, safinamide, selegiline, tranylcypromine), motion sickness medication (e.g., meclizine, scopolamine), potassium tablets/capsules, tricyclic antidepressants (e.g., amitriptyline, doxepin). 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, 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.

OVERDOSE: 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 their local poison control center at 1-800-222-1222. Canada residents can call a provincial poison control center. 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.