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.