<P> Common side effects include movement problems and nausea . More serious side effects include depression, low blood pressure with standing, sudden onset of sleepiness, psychosis, and increased risk taking behavior . Carbidopa prevents the breakdown of levodopa outside the brain . In the brain, levodopa is broken down into dopamine by which it has its effects . </P> <P> It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system . It is available as a generic medication and is moderately expensive . The wholesale price in the developing world is about 1.80 to 3.00 USD a month . In the United States a month's supply is about 50 to 150 USD . </P> <P> It is primarily used to improve the symptoms of Parkinson's disease but does not change the course of the disease . It can take two to three weeks of treatment before benefits are seen . Each dose then begins working in about ten minutes with a duration of effect of about five hours . </P> <P> A formulation that can be given in an intra-intestinal pump, known as Duodopa is being developed . </P>

Why is l-dopa usually given in combination with carbidopa
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