What Is the Relationship between Carbidopa and Levodopa?

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  • Written By: H. Colledge
  • Edited By: Heather Bailey
  • Last Modified Date: 09 November 2019
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Carbidopa and levodopa are drugs used in the treatment of Parkinson's disease. Parkinson's is a disease in which progressive changes take place in the brain, including loss of the nerve cells responsible for producing a substance called dopamine. Levodopa causes an increase in dopamine levels because it is converted into dopamine inside the brain. Carbidopa is normally taken together with levodopa because it stops levodopa conversion taking place outside the brain. This ensures the maximum amount of dopamine becomes available where it is needed, inside the brain, and reduces side effects caused by the effects of dopamine in other parts of the body.

In Parkinson's disease, part of the brain inside an area known as the basal ganglia becomes abnormal. As well as loss of those nerve cells which produce dopamine, clumps of protein called Lewy bodies develop. Although the effects of the brain changes are not fully understood, it is known that a lack of dopamine may be responsible for the movement problems seen in Parkinson's disease. Symptoms of Parkinson's typically include shaking, or tremor, stiffness, or rigidity, and slowness of movement. Together, these symptoms cause an individual to experience difficulty walking and problems carrying out everyday activities.


Other symptoms, which are not related to movement, can include pain, constipation and depression. Parkinson's usually progresses over a number of years and there is no cure, but medications such as carbidopa and levodopa can help control the condition. Levodopa is the main drug treatment used to reduce the symptoms of Parkinson's disease. It is taken in the form of tablets, absorbed in the gut and taken into the bloodstream. While dopamine itself is not able to cross the barrier between the blood and the brain, levodopa can do this and it is then converted into dopamine.

Unfortunately, levodopa is also converted into dopamine in other parts of the body, causing side effects such as vomiting and nausea. In order to prevent this from happening, carbidopa is administered at the same time. Typically carbidopa and levodopa are combined together in tablet form. Carbidopa acts to stop levodopa from being converted to dopamine in parts of the body outside the brain, reducing the risk of adverse effects. Even so, it is possible to experience problems such as nausea when first beginning to take carbidopa and levodopa, and to avoid this, the medication is usually increased gradually from a low starting dose.


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