Levodopa

This is synthetic dopamine given orally to boost their own dopamine levels. It is usually combined with a drug that stops levodopa being broken down in the body before it gets the chance to enter the brain. These are peripheral decarboxylase inhibitors. Examples are carbidopa and benserazide.

Combination drugs are:

• Co-benyldopa (levodopa and benserazide)
• Co-careldopa (levodopa and carbidopa)

Levodopa is the most effective treatment for symptoms but becomes less effective over time. It is often reserved for when other treatments are not managing to control symptoms.

The main side effect of dopamine is when the dose is too high patients develop dyskinesias. Theses are abnormal movements associated with excessive motor activity. Examples are:

• Dystonia: This is where excessive muscle contraction leads to abnormal postures or exaggerated movements.
• Chorea: These are abnormal involuntary movements that can be jerking and random.
• Athetosis: These are involuntary twisting or writhing movements usually in the fingers, hands or feet.

COMT Inhibitors

The main example of this is entacapone. These are inhibitors of catechol-o-methyltransferase  (COMT). The COMT enzyme metabolises levodopa in both the body and brain. Entacapone is taken with levodopa (and a decarboxylase inhibitor) to slow breakdown of the levodopa in the brain. It extends the effective duration of the levodopa.

Dopamine Agonists

These mimic dopamine in the basal ganglia and stimulate the dopamine receptors. They are less effective than levodopa in reducing symptoms. They are usually used to delay the use of levodopa and are then used in combination with levodopa to reduce the dose of levodopa that is required to control symptoms. One notable side effect with prolonged use is pulmonary fibrosis. Examples are:

• Bromocryptine
• Pergolide
• Carbergoline