What Is Carbidopa/Levodopa Therapy?

Dopamine is the neurotransmitter (chemical messenger) responsible for producing smooth, purposeful movement. When Parkinson’s disease (PD) damages the neurons in the brain that make dopamine, the motor symptoms of PD appear. This includes symptoms like:1-4

These motor symptoms are related to the depletion of dopamine in the brain. Research has shown that by the time motor symptoms of PD appear, about 60 to 80 percent of the neurons in the brain that make dopamine have already been damaged or destroyed.1-4

The combination of levodopa and carbidopa is the most effective treatment available to manage the motor symptoms of PD. Levodopa, also called L-dopa, was first discovered as a treatment for the symptoms of Parkinson’s disease more than 50 years ago.1-4

How does carbidopa/levodopa therapy work?

Giving dopamine as a therapy by itself is ineffective because it cannot cross the blood-brain barrier. However, levodopa can cross the blood-brain barrier and relieve the motor symptoms of PD. Levodopa is the precursor to dopamine. It is the substance used to make dopamine.1-4

Carbidopa is added to levodopa to prevent levodopa from being converted into dopamine in the bloodstream. This allows more of the drug to get to the brain. This also means that lower doses of levodopa can be given. The addition of carbidopa also reduces the risk of the side effects caused by dopamine.1,2

Examples of carbidopa/levodopa therapy

Several different carbidopa/levodopa therapies are used to treat PD, including:1

  • Sinemet® (carbidopa levodopa)
  • Sinemet® CR (carbidopa levodopa ER)
  • Parcopa® (carbidopa levodopa) orally disintegrating tablet
  • Stalevo® (carbidopa/levodopa/entacapone)
  • Rytary® (carbidopa levodopa ER)
  • Inbrija® (levodopa inhalation powder)
  • Duopa® (carbidopa levodopa) enteral suspension

There are different dosages for each of these therapies. People may be prescribed different dosages at different points in their disease to manage their symptoms.1

  • Sinemet® – A tablet that comes in 3 different strengths.
  • Sinemet® CR – A sustained-release tablet that releases the ingredients over a 4- to 6-hour timeframe. It comes in 2 different strengths.
  • Parcopa® – Taken by mouth and rapidly disintegrates on the tongue. No water is required to take Parcopa.
  • Stalevo® – A tablet that includes a combination of carbidopa, levodopa, and entacapone (a COMT inhibitor). Entacapone is believed to allow more levodopa to reach the brain by blocking the COMT enzyme that breaks down levodopa and dopamine.
  • Rytary® – An extended-release tablet that comes in 4 different strengths.
  • Inbrija® – A form of levodopa that can be inhaled when a person begins to have an "off" period.
  • Duopa® – A gel that is delivered directly into the intestines by a pump for up to 16 hours. It requires a procedure to make a small hole (stoma) in the stomach wall. It provides another option for people who still experience PD symptoms with oral treatments.

Side effects

The most common side effects of carbidopa/levodopa therapy include:1

  • Nausea
  • Vomiting
  • Orthostatic hypotension (falling blood pressure that occurs upon standing)
  • Worsening of glaucoma
  • Dyskinesia (spontaneous, involuntary movements)
  • Hallucination
  • Psychosis
  • Low blood pressure
  • Confusion
  • Dry mouth
  • Dizziness

Dyskinesias are a common side effect of long-term levodopa therapy. Dyskinesia can greatly affect quality of life, and some people find them very disturbing. The severity of dyskinesia due to levodopa therapy (also called levodopa-induced dyskinesia) varies among people with PD. The risk of dyskinesia is also higher in early-onset PD. For those who experience dyskinesia, medicines may be added or adjusted. Deep brain stimulation may also be an option.5,7

These are not all the possible side effects of carbidopa/levodopa therapy. Talk to your doctor about what to expect or if you experience any changes that concern you during treatment with carbidopa/levodopa.

Things to know about carbidopa/levodopa therapy

Additional therapy

In people whose symptoms of PD are not well controlled by carbidopa/levodopa therapy, more treatments may be added. People may also be switched to other drugs. Other treatments for PD include:6

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Written by: Emily Downward | Last reviewed: May 2021