Anti-Parkinsons drugs are intended primarily for treating Parkinson's disease. They are not psychiatric drugs, which means they are not licenced to treat any mental health problems. However they may be prescribed alongside an antipsychotic to reduce neuromuscular side effects which resemble symptoms of Parkinson’s disease.

The three anti-Parkinson's drugs which may be prescribed with antipsychotics are:

• orphenadine (Biorphen, Disipal)
• procyclidine (Arpicolin, Kemadrin)
• trihexyphenidyl (formerly called benzhexol)

Anti-Parkinson's drugs are sometimes referred to as 'antimuscarinics' because their main side effects are antimuscarinic. There are no significant differences between these drugs, but you may find that you tolerate one better than another.

When might I be prescribed one of these drugs?

You should only be prescribed an anti-Parkinson's drug if you have actually developed Parkinsonism as a side effect of your antipsychotic and:

• you can't switch to a different antipsychotic
• you can't reduce your dose
• changing the drug or reducing the dose has not helped your Parkinsonism symptoms

These drugs should never be prescribed to prevent side effects from occurring if you haven't already experienced any.

What are the risks with these drugs?

• They can cause side effects (see below for details of possible side effects each of these three drugs can cause).
• When coming off them, you may get rebound Parkinson's symptoms if you stop taking them too suddenly – so it's important to reduce your dose gradually. If you are taking one of these drugs with an antipsychotic and you are coming off the antipsychotic, withdrawal guidelines say that it's best to come off the antipsychotic first and then to come off the anti-Parkinson's drug.
• They have a stimulant effect, which means they can occasionally be habit-forming.

You should be particularly cautious about taking these drugs if you have:

• a heart condition
• high blood pressure
• liver disease
• kidney disease

You should try to avoid these drugs if you:

• have glaucoma (a serious eye condition), or are at risk of it
• are showing signs of tardive dyskinesia
• have myesthenia gravis (a rare, serious muscle disorder)
• are pregnant or breastfeeding – this is because there's very little information on how safe these drugs are, and babies are sensitive to antimuscarinic effects
• have an enlarged prostate

About orphenadrine

Key facts

• Trade names: Biorphen, Disipal
• Prescribed for: Parkinsonism; movement disorders and other Parkinson's symptoms caused by antipsychotics
• Usual daily dose: 100–300 mg
• Forms available: tablets, liquid
• Half-life: about 14 hours
• tablets contain lactose and gelatin

Possible side effects

About procyclidine

Key facts

• Trade names: Arpicolin, Kemadrin
• Prescribed for: Parkinsonism, movement disorders and other Parkinson's symptoms caused by antipsychotics
• Usual daily dose: 10–30mg
• Forms available: tablets, liquid
• Half-life: three to four hours
• tablets contain lactose

Possible side effects

About trihexyphenidyl

Key facts

• Trade names: Agitane, Artane, Benzhexol
• Prescribed for: Parkinsonism, movement disorders and other Parkinson's symptoms caused by antipsychotics
• Usual daily dose: maximum 20mg
• Forms available: tablets, liquid
• Half-life: three to four hours

Possible side effects

Unfortunately, because this drug was first licensed before the current system of recording side effects was widely used, estimates of how likely you are to experience different side effects are not available for trihexyphenidyl. All side effects are listed here in alphabetical order, but you might find them listed in order of how common they are in the patient information leaflet (PIL) – the leaflet that comes in the drug packet.

This information was published in 2016. We will revise it in 2018.