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Trihexyphenidyl is usually prescribed to relieve unwanted side-effects caused by certain other medicines.

When starting your treatment your doctor will give you a small dose and will then gradually increase your dose every few days. This helps your body adjust to the medicine and reduces the risk of side-effects. Take it exactly as your doctor tells you to.

The most common side-effects are a dry mouth, constipation, blurred vision, and feeling sick (nausea).

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About trihexyphenidyl

Type of medicine

An antimuscarinic medicine

Used for

Unwanted 'extrapyramidal side-effects' caused by some medicines

Available as

Tablets and oral liquid medicine

Trihexyphenidyl is an antimuscarinic medicine. It is prescribed for people who have unwanted movement disorders as a side-effect of taking certain other medicines, such as some antipsychotic medicines. These movement disorders are often referred to as 'extrapyramidal side-effects' and include things like uncontrolled face and body movements, tremor, and restlessness.

Trihexyphenidyl can also be given to relieve muscle stiffness in people with Parkinson's disease, although other medicines are generally preferred.

Before taking trihexyphenidyl

Some medicines are not suitable for people with certain conditions, and sometimes a medicine may only be used if extra care is taken. For these reasons, before you start taking trihexyphenidyl it is important that your doctor knows:

  • If you are pregnant, trying for a baby or breastfeeding.

  • If you have any problems with the way your liver works, or the way your kidneys work.

  • If you have any problems with your heart or blood vessels.

  • If you have prostate problems, or if you have been experiencing difficulty passing urine.

  • If you have been constipated for more than a week.

  • If you have a condition which causes raised pressure in your eyes, such as glaucoma.

  • If you have high blood pressure.

  • If you have a condition causing muscle weakness, called myasthenia gravis.

  • If you have ever had a mental health problem called psychosis.

  • If you are taking any other medicines. This includes any medicines you are taking which are available to buy without a prescription, as well as herbal and complementary medicines.

  • If you have ever had an allergic reaction to a medicine.

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How to take trihexyphenidyl

  • Before you start this treatment, read the manufacturer's printed information leaflet from inside your pack. It will give you more information about trihexyphenidyl and will provide you with a full list of the side-effects which you may experience from taking it.

  • Take trihexyphenidyl exactly as your doctor tells you to. When starting your treatment your doctor will give you a small dose (usually 1 mg daily) and will then gradually increase your dose every few days. Carefully follow the directions your doctor gives to you. 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 tablets should be taken with or just after a meal. Try to take your doses at the same times of day each day, as this will help you to remember to take your doses regularly.

  • If you forget to take a dose, take it as soon as you remember unless your next dose is due. If it is nearly time for your next dose then take the next dose when it is due but leave out the forgotten dose. Do not take two doses together to make up for a missed dose.

Getting the most from your treatment

  • Try to keep your regular appointments with your doctor. This is so your doctor can check on your progress.

  • If you buy any medicines, check with a pharmacist that they are safe to take with trihexyphenidyl. Some antihistamines and some strong painkillers can interfere with trihexyphenidyl and increase the risk of side-effects.

  • If you are having an operation or any dental treatment, tell the person carrying out the treatment that you are taking trihexyphenidyl.

  • If you have been taking trihexyphenidyl for some time, do not stop taking it without speaking with your doctor first. Stopping suddenly can cause problems so your doctor will want to reduce your dose gradually if this becomes necessary.

  • Ask your doctor for advice before drinking alcohol while you are on trihexyphenidyl. Your doctor may recommend you do not drink alcohol because it increases the possibility of side-effects such as feeling sleepy.

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Can trihexyphenidyl cause problems?

Along with their useful effects, most medicines can cause unwanted side-effects although not everyone experiences them. The table below contains some of the most common ones associated with trihexyphenidyl. You will find a full list in the manufacturer's information leaflet supplied with your medicine. The unwanted effects usually improve as your body adjusts to the new medicine, but speak with your doctor or pharmacist if any of the following continue or become troublesome.


trihexyphenidyl side-effects (these affect fewer than 1 in 10 people)

What can I do if I experience this?

Dry mouth

Try chewing sugar-free gum or sucking sugar-free sweets. If a dry mouth is still troublesome, speak with your pharmacist for advice

Feeling sick (nausea) or being sick (vomiting)

Stick to simple foods - avoid rich or spicy meals

Blurred vision, feeling dizzy

If this happens, do not drive and do not use tools or machines


Try to eat a well-balanced diet and drink several glasses of water each day

Difficulty passing urine

If this becomes troublesome, speak with your doctor for advice

If you experience any other symptoms which you think may be due to the medicine, speak with your doctor or pharmacist for further advice.

How to store trihexyphenidyl

  • Keep all medicines out of the reach and sight of children.

  • Store in a cool, dry place, away from direct heat and light.

Important information about all medicines

Important information about all medicines

Never take more than the prescribed dose. If you suspect that you or someone else might have taken an overdose of this medicine, go to the accident and emergency department of your local hospital. Take the container with you, even if it is empty.

This medicine is for you. Never give it to other people even if their condition appears to be the same as yours.

Do not keep out-of-date or unwanted medicines. Take them to your local pharmacy which will dispose of them for you.

If you have any questions about this medicine ask your pharmacist.

Report side effects to a medicine or vaccine

If you experience side effects, you can report them online through the Yellow Card website.

Further reading and references

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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