After five to 10 years of taking a levodopa preparation, between 50 and 70 per cent of people experience ON or OFF episodes. An 'ON' is when they can perform activities of daily living as normal for them while an 'OFF' is when they completely freeze and voluntary movement is difficult or impossible. This can be extremely frightening and may cause a great degree of anxiety and concern for the person with PD and their families. These fluctuations in symptom control may not necessarily be related to timing of medicines administration.

Some 85 per cent of people with PD who take levodopa experience the agent's efficacy 'wearing off', while 37 per cent experience a sudden ON/OFF and 34 per cent a delayed response to usual treatment.

At this stage, an adjuvant agent is required to reduce complications and improve quality of life. There is no single adjuvant agent of choice and selection is based on patient preference after the short- and long-term benefits and drawbacks of each class of medicine have been explained to them.

If a person has been on a levodopa preparation first line, either a COMT inhibitor or a non-ergot derived dopamine agonist could be introduced. In the latter scenario, the dose of levodopa must be lowered to prevent dyskinesia (difficulty or distortion in performing voluntary movements) and neuropsychiatric effects such as hallucinations and psychosis.

Conversely, if a person had been on a non-ergot derived dopamine agonist first line then either a levodopa preparation or a COMT inhibitor could be added.