What is trihexyphenidyl used for?

• Reducing abnormal body movements that are side effects of antipsychotic medicines (these are sometimes called extrapyramidal symptoms and may include involuntary face and body movements, restlessness and tremor).
• Parkinson's disease (trihexyphenidyl can help with tremor, rigidity and excessive salivation).
• This medicine is not licensed for use in children under 18 years of age, though it may sometimes be prescribed by specialists.

How does trihexyphenidyl work?

• Trihexyphenidyl hydrochloride is a type of medicine called an antimuscarinic. Antimuscarinic medicines (sometimes also called anticholinergics) work by preventing the activity of a neurotransmitter called acetylcholine.
• Neurotransmitters are chemical compounds that are found in the brain and nerves, and act as chemical messengers between the nerve cells. There are various different neurotransmitters in the body, with various different functions. Several diseases and conditions involve either overactivity or underactivity of certain neurotransmitters.
• In Parkinson's disease there is a deficiency in the brain of a neurotransmitter called dopamine. Normally there is a balance between the activity of dopamine and the activity of acetylcholine in the brain. However, in Parkinson's disease, the dopamine deficiency results in overactivity of acetylcholine. This causes some of the symptoms of the disease.
• By contrast, some psychiatric illnesses, for example schizophrenia, are associated with overactivity of dopamine in the brain. Many antipsychotic medicines that are used to treat these diseases work by decreasing dopamine activity in the brain. However, because they decrease dopamine they can produce side effects that resemble the symptoms of Parkinson's disease.
• Trihexyphenidyl helps control Parkinson's type symptoms by blocking the receptors that acetylcholine acts on and thus reducing acetylcholine activity. This helps restore the balance of acetylcholine and dopamine in the brain.

How do I take trihexyphenidyl?

• Always follow the instructions that your doctor has given you. Trihexyphenidyl is usually started as a low dose that is gradually increased every few days until your symptoms are controlled. Carefully follow your doctor's directions. Once you are on a full dose, it is likely that you will be taking three doses of trihexyphenidyl a day, although it may be more or less often than this.
• Trihexyphenidyl is available as tablets and syrup. The tablets should be taken just before or just after a meal.
• If you forget to take a dose at your usual time, take it as soon as you remember, unless it is nearly time for your next dose. In this case, leave out the forgotten dose and just take your next dose as usual. Do not take a double dose to make up for a missed dose.
• Don't suddenly stop taking trihexyphenidyl if you have been taking it for a long time, as this could make  your symptoms come back. If treatment is to be stopped, this should be done gradually, following the instructions given by your doctor or pharmacist.

What should I know before taking trihexyphenidyl?

• Trihexyphenidyl may cause blurred vision, confusion or reduced alertness. You should avoid performing potentially hazardous tasks such as driving or operating machinery until you know how trihexyphenidyl affects you and are sure you can do so safely.

Who should not take trihexyphenidyl?

• Trihexyphenidyl should not be used to treat a condition called tardive dyskinesia. This condition is a potential side effect of antipsychotic medicines and involves rhythmical involuntary movements of the tongue, face, mouth and jaw, sometimes accompanied by involuntary movements of the arms and legs.
• Trihexyphenidyl tablets and syrup should not be used if you are allergic to any of their ingredients. Please inform your doctor or pharmacist if you have previously experienced such an allergy. If you feel you have experienced an allergic reaction, stop taking trihexyphenidyl and inform your doctor or pharmacist immediately.

Trihexyphenidyl should be used with caution in

• Elderly people.
• People with liver or kidney problems.
• People with heart disease or high blood pressure (hypertension).
• People with glaucoma.
• People who have any difficulties passing urine, for example men with an enlarged prostate gland (prostatic hypertrophy).
• People who are constipated.
• People with a condition called myasthenia gravis in which there is abnormal muscle weakness.

Can I take trihexyphenidyl while pregnant or breastfeeding?

• The safety of trihexyphenidyl for use during pregnancy has not been established. It should not be used during pregnancy unless considered essential by your doctor. Get further medical advice from your doctor.
• It is not known if trihexyphenidyl passes into breast milk. It may affect the production of breast milk and should not be used by women who are breastfeeding. Ask your doctor for further advice.

What are the possible side effects of trihexyphenidyl?

• Dry mouth.
• Feeling thirsty.
• Constipation.
• Feeling sick.
• Blurred vision.
• Dizziness.
• Dry skin, flushing or skin rashes.
• Difficulty passing urine.
• Confusion.
• Memory problems.
• Agitation.
• Restlessness.
• Difficulty sleeping (insomnia).
• Seeing or hearing things that are not really there (hallucinations).
• A state of well-being, optimism and cheerfulness (euphoria).
• Raised pressure in the eyeball.

Talk to your doctor, nurse or pharmacist if you want any more information about the possible side effects of trihexyphenidyl. 

Can I take other medicines with trihexyphenidyl?

Many groups of medicines have antimuscarinic effects similar to trihexyphenidyl. If you take any of these with trihexyphenidyl you are more likely to experience side effects such as blurred vision, dry mouth, constipation, difficulty passing urine and confusion. Other medicines with antimuscarinic effects include the following:

• amantadine
• antispasmodics, eg hyoscine, atropine
• antipsychotics, eg chlorpromazine, clozapine, haloperidol
• certain antisickness medicines, eg promethazine, prochlorperazine, meclozine, cyclizine
• codeine
• disopyramide
• MAOI antidepressants, eg phenelzine
• medicines for urinary incontinence, eg oxybutynin, flavoxate, tolterodine, propiverine, trospium
• nefopam
• other antimuscarinics for Parkinson's symptoms eg procyclidine, orphenadrine
• sedating antihistamines, eg chlorphenamine
• tricyclic or related antidepressants, eg amitriptyline, maprotiline.

If you get a dry mouth while taking trihexyphenidyl you may find that medicines that are designed to dissolve and be absorbed from under the tongue, eg sublingual glyceryl trinitrate (GTN) tablets, become less effective. This is because the tablets do not dissolve properly in a dry mouth. To resolve this, drink a mouthful of water before taking sublingual tablets.

Trihexyphenidyl may reduce the absorption of levodopa from the gut. If you are taking levodopa (in co-beneldopa or co-careldopa) and start taking trihexyphenidyl as well, let your doctor know if your levodopa seems to be less effective.

Trihexyphenidyl may reduce the effects of the following medicines on the gut:

• cisapride
• domperidone
• metoclopramide.

Trihexyphenidyl may have an opposite effect to cholinergic medicines, which work by increasing the activity of acetylcholine. If trihexyphenidyl is used in combination with medicines that have cholinergic effects, the medicines may oppose each others effects, making one or both less effective. Medicines with cholinergic effects include the following:

• medicines for myasthenia gravis, eg neostigmine, edrophonium, distigmine, pyridostigmine
• medicines for Alzheimer's disease, eg galantamine, donepezil, rivastigmine
• tacrine.

References:

https://www.medicines.org.uk/emc/product/5271/smpc

https://www.netdoctor.co.uk/medicines/brain-nervous-system/a6336/trihexyphenidyl/

https://www.webmd.com/drugs/2/drug-6683/artane-oral/details

https://www.drugs.com/mtm/artane.html

https://www.rxlist.com/artane-side-effects-drug-center.htm

http://mcs.open.ac.uk/nlg/old_projects/pills/corpus/PIL/data/Wyeth/Artane/Artane.html