Carbidopa/Levodopa

Levodopa and carbidopa are used in combination forms to treat symptoms of Parkinson's disease.  Levodopa is a metabolic precursor to dopamine that is converted to dopamine in the substantia nigra region of the brain to replace the deficit of naturally produced dopamine, which is the source of Parkinson's symptoms.  Levodopa is converted to dopamine by DOPA decarboxylase.

Carbidopa is a decarboxylase inhibitor that acts peripherally. Use of carbidopa in combination with levodopa allows for smaller doses of dopamine because levodopa is not converted to dopamine as readily in peripheral tissues due to the inhibition caused by carbidopa of the decarboxylase enzymes.  This allows for fewer side-effects of administration of levodopa.

Class Drugs

• Levodopa/Carbidopa

Class Mechanism of Action

Levodopa

• Metabolic precursor to dopamine, converted to dopamine in substantia nigra

Carbidopa

• Inhibits dopamine decarboxylase in peripheral tissues.

• Allows greater therapeutic effect at a smaller dose of levodopa, thus diminishing side effects.

Class Application

Parkinson's Disease

• Decreases rigidity and tremors.

• Most useful in the first 3 years of symptoms

• As the number of neurons in the substantia nigra decreases, levodopa/carbidopa becomes less effective.

Class Adverse Effects

Peripheral Effects (dopamine mediated effects)

• Anorexia

• Nausea

• Tachycardia

• Hypotension

• Brown urine (catecholamine oxidation produces melanin pigment)

• Hypertensive crisis when administered with MAOi

• Arrhythmias

Central Effects

• Visual and auditory hallucinations, paranoia, and episodes of confusion tend to worsen later in treatment (not treated with typical antipsychotics, which tend to worsen Parkinson's features).

• Dyskinesia (over-activity of dopamine in the basal ganglia)

Drug Table

Drug Name	Application	Comments	Image
Levodopa	Parkinson's Disease	Contraindications:; History of psychosis; Narrow-angle glaucoma; Peptic ulcer disease	"Lever" and "Dope"
Carbidopa	Parkinson's Disease		"Car" and "Dope"