Why is carbidopa always given with levodopa

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When levodopa was first introduced, its major side effects were nausea and vomiting. In spite of its remarkable benefits, many patients could not or would not take it. The problem was in the conversion of L-dopa or levodopa to dopamine. The L-dopa or levodopa was changed in the stomach by an enzyme called dopa-decarboxylase (DDC) into dopamine. Dopamine in the stomach and circulating in the blood cannot cross the blood/brain barrier to reach the sub-stantia nigra. However, dopamine in the stomach and circulating in the blood was reacting with the brain's trigger zone for nausea. To prevent the levodopa from being changed to dopamine outside the brain, car-bidopa was added as a way to block the enzyme DDC. Carbidopa does not pass through the blood/brain barrier and insures that the conversion of levodopa to dopamine will take place in the substantia nigra, well past the nausea trigger zone. The addition of car-bidopa prevents nausea and allows a smaller amount of levodopa to be used. Before the introduction of car-bidopa the average dose of levodopa was 2000 to 4000 mg a day. With the addition of carbidopa the average dose of levodopa is 300 to 600 mg a day.

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