carbidopa-levodopa

Action

After conversion to dopamine in CNS, levodopa acts as a neurotransmitter, relieving symptoms of Parkinson's disease. Carbidopa prevents destruction of levodopa, making more levodopa available to be decarboxylated to dopamine in brain.

Availability

Tablets: 10 mg carbidopa/100 mg levodopa, 25 mg carbidopa/100 mg levodopa, 25 mg carbidopa/250 mg levodopa

Tablets (extended-release): 25 mg carbidopa/100 mg levodopa, 50 mg carbidopa/200 mg levodopa

Indications and dosages

➣ Idiopathic Parkinson's disease; parkinsonism; symptomatic parkinsonism

Conventional tablets-

Adults not currently receiving levodopa: Initially, 10 mg carbidopa/100 mg levodopa P.O. three to four times daily or 25 mg carbidopa/100 mg levodopa t.i.d.; may be increased q 1 to 2 days until desired effect occurs

Adults converting from levodopa alone (less than 1.5 g/day): Initially, 25 mg carbidopa/100 mg levodopa three to four times daily; may be increased q 1 to 2 days until desired effect occurs

Adults converting from levodopa alone (more than 1.5 g/day): Initially, 25 mg carbidopa/250 mg levodopa three to four times daily; may be increased q 1 to 2 days until desired effect occurs

Extended-release tablets-

Adults not currently receiving levodopa: Initially, 50 mg carbidopa/200 mg levodopa P.O. b.i.d., with doses spaced at least 6 hours apart

Adults converting from standard carbidopa-levodopa: Initiate therapy with at least 10% more levodopa content/day (may need up to 30% more) given at 4- to 8-hour intervals while awake; wait 3 days between dosage changes. Some patients may need higher dosages and shorter dosing intervals.

Contraindications

• Hypersensitivity to drug or tartrazine

• Angle-closure glaucoma

• MAO inhibitor use within past 14 days

• Malignant melanoma

• Breastfeeding

Precautions

Use cautiously in:

• cerebrovascular, renal, hepatic, or endocrine disease

• history of cardiac, psychiatric, or ulcer disease

• abrupt drug discontinuation or dosage

• pregnant patients

• children ages 18 and under (safety not established).

Administration

• Give dose as close as possible to time ordered to ensure stable drug blood level.

• Know that giving extended-release form with food increases drug bioavailability.

• If patient needs general anesthesia, continue drug therapy as appropriate (if he's allowed to have oral fluids and drugs).

See Be aware that drug shouldn't be withdrawn abruptly.

Adverse reactions

CNS: anxiety, dizziness, hallucinations, memory loss, headache, numbness, confusion, insomnia, nightmares, delusions, psychotic changes, depression, dementia, poor coordination, worsening hand tremor

CV: cardiac irregularities, palpitations, orthostatic hypotension

EENT: blurred vision, diplopia, mydriasis, eyelid twitching, difficulty swallowing

GI: nausea, vomiting, diarrhea, constipation, abdominal pain or discomfort, flatulence, excessive salivation, dry mouth, anorexia, upper GI hemorrhage (with history of peptic ulcer)

GU: urinary retention, urinary incontinence, dark urine

Hematologic: hemolytic anemia, leukopenia

Hepatic: hepatotoxicity

Musculoskeletal: muscle twitching, involuntary or spasmodic movements

Respiratory: hyperventilation

Skin: melanoma, flushing, rash, abnormally dark sweat

Other: altered or bitter taste, burning sensation of tongue, tooth grinding (especially at night), weight changes, hot flashes, hiccups

Interactions

Drug-drug. Anticholinergics: decreased carbidopa-levodopa absorption

Antihypertensives: additive hypotension

Haloperidol, papaverine, phenothiazines, phenytoin, reserpine: reversal of carbidopa-levodopa effects

Inhalation hydrocarbon anesthetics: increased risk of arrhythmias

MAO inhibitors: hypertensive reactions

Methyldopa: altered efficacy of carbidopa-levodopa, increased risk of adverse CNS reactions

Pyridoxine: antagonism of carbidopa-levodopa effects

Selegiline: increased risk of adverse reactions

Drug-diagnostic tests. Alanine aminotransferase, alkaline phosphatase, aspartate aminotransferase, bilirubin, blood urea nitrogen, lactate dehydrogenase, low-density lipoproteins, protein-bound iodine, uric acid: increased levels
Coombs' test: false-positive result

Granulocytes, hemoglobin, platelets, white blood cells: decreased values

Urine glucose, urine ketones: test interference

Drug-food. Foods rich in pyridoxine (liver, yeast, cereals): reversal of carbidopa-levodopa effects

Drug-herbs. Kava: decreased carbidopa-levodopa efficacy

Octacosanol: worsening of dyskinesia

Drug-behaviors. Cocaine use: increased risk of adverse reactions to carbidopa-levodopa

Patient monitoring

• Monitor patient for orthostatic hypotension.

• Assess patient's need for drug "holiday" if his response to drug decreases.

Patient teaching

See Inform patient that muscle and eyelid twitching may indicate toxicity. Tell him to report these symptoms immediately.

See Caution patient not to stop taking drug abruptly.

• Instruct patient to swallow extended-release tablets whole without crushing or chewing them.

• Advise patient to move slowly when sitting up or standing, to avoid dizziness or light-headedness caused by sudden blood pressure drop.

• Tell patient that drug may darken or discolor his urine and sweat.

• As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs, tests, foods, herbs, and behaviors mentioned above.