Dopamine agonists stimulate dopamine receptors both post- and presynaptically and have a similar but less potent effect than levodopa. They are classified as ergot derived (bromocriptine, pergolide and cabergoline) or non-ergot derived (apomorphine, pramipexole, rotigotine and ropinirole). Ergot derived dopamine agonists are often associated with a risk of moderate to severe cardiac valvulopathy and serosal fibrosis, so are not first-line treatments. 

For some people, dopamine agonists and the development of compulsive behaviours (addictive gambling and an excessive increased libido) have been linked, especially at high doses. Sudden onset of sleep also occurs more frequently, particularly when the dose is being increased, so patients should be advised to avoid driving and/or operating machinery while the dose is being upped if this side-effect occurs. 

MAO-B inhibitors, such as selegiline and rasagiline, prevent the breakdown of dopamine, so have a levodopa-sparing effect. They typically provide less improvement than levodopa in motor symptoms and daily functioning, and can be used in the early stages of the disease as monotherapy or an adjunct to other medications. 

If used adjunctly, MAO-B inhibitors help to reduce ‘off’ periods and to extend ‘on’ periods. 

Triptans or selective serotonin reuptake inhibitors should not be used concurrently due to the increased risk of potentially fatal serotonin syndrome. When advising on over-the-counter decongestants or cold remedies, people who are on MAO-B inhibitors should be advised to not take medications that contain pseudoephedrine due to the increased risk of hypertensive crisis.

There are also some prescribed medications that people with PD should avoid. Medication that inhibits dopamine receptors could worsen symptoms of the disease. For example, typical antipsychotic medications (e.g. phenothiazines and haloperidol) exacerbate the motor symptoms of PD, as do antiemetics such as prochlorperazine and metoclopramide.