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