Medically Reviewed on 11/28/2022

Generic Name: carbidopa

Brand Name: Lodosyn

Drug Class: Decarboxylase Inhibitors

What is carbidopa, and what is it used for?

Carbidopa is a medication always used in combination with levodopa used to treat Parkinson’s disease, a degenerative movement disorder.

Carbidopa is used to increase the bioavailability of levodopa by preventing it from breaking down before it reaches the central nervous system (CNS). This helps in enhancing the CNS effects of levodopa with a relatively lower dosage, while reducing its peripheral side effects such as nausea and vomiting.

Carbidopa belongs to a class of medications known as decarboxylase inhibitors. Carbidopa works by inhibiting the aromatic amino-acid decarboxylase enzyme which converts levodopa into dopamine. Carbidopa also inhibits the metabolism of levodopa in the gastrointestinal tract. These two actions prevent the breakdown of levodopa into dopamine peripherally and allow the maximum amount of drug to reach the brain.

Dopamine cannot cross the blood-brain barrier, whereas levodopa can get into the brain tissue where it gets converted into dopamine and exerts its effects. Dopamine is an important chemical (neurotransmitter) with multiple functions including the regulation of movement, memory, and emotions. Carbidopa cannot cross the blood-brain barrier, hence its effects on decarboxylase enzyme is only peripheral and it does not affect dopamine synthesis in the brain.

Carbidopa is used with levodopa or carbidopa-levodopa combination drug for Parkinson's disease in patients:

  • For whom the dosage of carbidopa-levodopa combo provides less than adequate daily dosage of carbidopa
  • Whose dosage requirement of carbidopa and levodopa necessitates separate titration of each entity


  • Do not use in patients with hypersensitivity to carbidopa or any of its components.
  • Do not use carbidopa-levodopa combination in patients with narrow-angle glaucoma, a condition with high intraocular pressure that damages the optic nerve.
  • Do not use carbidopa-levodopa combination concurrently or for two weeks after treatment with monoamine oxidase inhibitor (MAOI) antidepressants.
  • Carbidopa does not have any antiparkinsonian effect as a single agent, and must always be administered with levodopa.
  • Carbidopa can reduce peripheral side effects of levodopa, but it does not decrease its CNS adverse reactions. The increased bioavailability in the CNS may result in certain CNS-associated adverse effects with smaller doses of levodopa.
  • When carbidopa is initiated in a patient receiving levodopa, the two drugs should be given at the same time. At least twelve hours should elapse between the last dose of levodopa and initiation of therapy with carbidopa-levodopa combination.
  • There are reports of patients on antiparkinsonian treatment suddenly falling asleep during daily activities without feeling sleepy prior to it. Caution patients appropriately.
  • Rare cases of symptom complex similar to neuroleptic malignant syndrome (NMS) have been reported with dose reduction or discontinuation of levodopa and carbidopa-levodopa. NMS is a life-threatening drug reaction to antipsychotic drugs, with symptoms including high fever, muscle rigidity and altered mental status. Monitor patients carefully when abruptly reducing dosage or discontinuing carbidopa-levodopa, especially patients on neuroleptic drugs, and treat appropriately if symptoms develop.
  • Patients treated with anti-Parkinson’s medications may experience intense sexual urges and uncontrollable urges to gamble or spend money. Caution the patients and their caregivers, and reduce dosage or discontinue treatment, if necessary.
  • Carbidopa-levodopa treatment has been associated with hallucinations and psychosis. Avoid use in patients with a major psychotic disorder, it may exacerbate psychosis.
  • Carbidopa can potentiate dopaminergic side effects of levodopa and may cause new onset or exacerbation of movement disorders (dyskinesia).
  • Monitor patients for development of depression and suicidal tendencies.
  • Studies show that patients with Parkinson’s disease have a higher risk for developing melanoma, a type of skin cancer. It is not clear whether it is due to the condition or the treatment, monitor Parkinson’s patients periodically for melanoma.


Parkinson's disease is only seen in people of advanced age. See Answer

What are the side effects of carbidopa?

Side effects have been observed only with concomitant use of carbidopa with levodopa or carbidopa/levodopa combination.

Common side effects include:

Call your doctor immediately if you experience any of the following symptoms or serious side effects while using this drug:

  • Serious heart symptoms include fast or pounding heartbeats, fluttering in your chest, shortness of breath, and sudden dizziness;
  • Severe headache, confusion, slurred speech, severe weakness, vomiting, loss of coordination, feeling unsteady;
  • Severe nervous system reaction with very stiff muscles, high fever, sweating, confusion, fast or uneven heartbeats, tremors, and feeling like you might pass out; or
  • Serious eye symptoms include blurred vision, tunnel vision, eye pain or swelling, or seeing halos around lights.

This is not a complete list of all side effects or adverse reactions that may occur from the use of this drug. Call your doctor for medical advice about serious side effects or adverse reactions. You may also report side effects or health problems to the FDA at 1-800-FDA-1088.

What are the dosages of carbidopa?


  • 25 mg


Parkinson's Disease

  • For use with carbidopa-levodopa or with levodopa for Parkinson's disease in patients for whom the dosage of carbidopa-levodopa combo provides less than adequate daily dosage of carbidopa, or in patients whose dosage requirement of carbidopa and levodopa necessitates separate titration of each entity
  • Dosage dependent upon concurrent dose of carbidopa-levodopa or levodopa and patient's specific needs
  • Patients taking Sinemet 10/100: Administer 25 mg carbidopa daily with first daily dose of Sinemet 10/100 if necessary; may administer 12.5-25 mg carbidopa with each subsequent dose of Sinemet 10/100; not to exceed 200 mg carbidopa/day including carbidopa from Sinemet
  • Patients taking Sinemet 25/250 or 25/100: Administer 25 mg carbidopa daily with either Sinemet strength throughout the day; not to exceed 200 mg/day of carbidopa including the Sinemet combination
  • Titration of carbidopa and levodopa (individually)
    • Administer 25 mg carbidopa orally every 6-8 hours concurrently with levodopa; administer 20-25% less than the previous levodopa in carbidopa naive patients; first dose of carbidopa should be taken 12 hours or earlier after the test dose of levodopa in carbidopa naive patients; increase or decrease dose by 1/2 or 1 tablet/day


  • Safety and efficacy not established


  • There are no reports of carbidopa overdose. Carbidopa overdose may be treated with supportive measures including gastric lavage to eliminate undigested drug, intravenous fluid administration, and maintenance of airway.
  • Patient should be continuously observed with electrographic monitoring for signs of irregular heart rhythm and appropriate treatment instituted, if necessary.

What drugs interact with carbidopa?

Inform your doctor of all medications you are currently taking, who can advise you on any possible drug interactions. Never begin taking, suddenly discontinue, or change the dosage of any medication without your doctor’s recommendation.

  • Carbidopa has no known severe or serious interactions with other drugs.
  • Carbidopa has moderate interactions with 71 different drugs.
  • Mild interactions of carbidopa include:
    • ferric maltol
    • ferrous fumarate
    • ferrous gluconate
    • ferrous sulfate
    • iron dextran complex
    • polysaccharide iron

The drug interactions listed above are not all of the possible interactions or adverse effects. For more information on drug interactions, visit the RxList Drug Interaction Checker.

It is important to always tell your doctor, pharmacist, or health care provider of all prescription and over-the-counter medications you use, as well as the dosage for each, and keep a list of the information. Check with your doctor or health care provider if you have any questions about the medication.

Pregnancy and breastfeeding

  • There are no adequate and well-controlled studies of carbidopa use in pregnant women. Carbidopa should be used during pregnancy only if maternal benefits outweigh the potential risks to the fetus.
  • It is not known if carbidopa is present in breast milk. Many drugs are present in breast milk and decision to discontinue the drug or nursing should be made taking into account the importance of the treatment to the mother.

What else should I know about carbidopa?

  • Take carbidopa exactly as prescribed by your physician. Do not change the dosage regimen of carbidopa and levodopa without consulting with your physician.
  • You may sometimes experience drug wearing-off effect towards the end of dosing interval. Notify your physician if you have such response and it bothers you.
  • Carbidopa-levodopa may cause sudden onset of drowsiness and sleep. Avoid hazardous tasks such as driving and operating heavy machinery while on treatment.
  • You may experience intense and uncontrollable sexual and other urges such as gambling and profligate spending while on carbidopa-levodopa treatment. Patients and caregivers should be alert for such urges and inform the healthcare provider.
  • Carbidopa-levodopa may cause a dark color (red, brown or black) to appear in sweat, urine or saliva. Although the color appears to be clinically unimportant, it may stain garments.
  • Store carbidopa safely out of reach of children.
  • In case of overdose, seek immediate medical help or contact Poison Control.

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Carbidopa is a medication always used in combination with levodopa used to treat Parkinson’s disease, a degenerative movement disorder. Common side effects with levodopa or carbidopa/levodopa combination include uncontrolled movements, impairment of voluntary movement control and slowness of movement, delusions, paranoid ideation, hallucinations, confusion, agitation, drowsiness (somnolence), dream abnormalities, nightmares, insomnia, and others.

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You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.

Medically Reviewed on 11/28/2022