When you take levodopa, it is converted into dopamine in the brain. This replaces the lost dopamine and therefore reduces some of the symptoms of the disease. Levodopa is also converted into dopamine in the rest of the body, which can cause unwanted side effects such as nausea and palpitations. Carbidopa is used in combination with the levodopa to prevent this happening. Carbidopa is a type of medicine called a dopa-decarboxylase inhibitor. It blocks the conversion of levodopa to dopamine in the body and so prevents these side effects. (Carbidopa cannot pass into the brain and so does not affect the conversion of levodopa to dopamine in the brain.)