levodopa and dopamine decarboxylase inhibitors

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L-DOPA is a precursor of dopamine. Dopamine iteself does not cross the blood-brain barrier and so is no effective as a drug. L-DOPA does enter the brain and is converted to dopamine in the striatum.

L-DOPA is usually given in conjunction with an inhibitor of dopamine decarboxylase. The inhibitor does not cross the blood-brain barrier hence the side-effects of peripheral dopamine production, such as nausea, are reduced while the central actions of L-DOPA are augmented.

NICE state that it is not possible to identify a universal first-choice drug therapy for people with early Parkinson's disease (PD). A possible inititial first-choice therapy is levodopa therapy (2)

  • the dose of levodopa should be kept as low as possible to maintain good function in order to reduce the development of motor complications
  • modified-release levodopa preparations may be used to reduce motor complications in people with later PD, but should not be drugs of first choice


  1. Clarke CE (1999). Managing early Parkinson's disease. The Practitioner; 243: 39-47.
  2. NICE (June 2006). Parkinson's disease

Last reviewed 04/2018