A fixed-dose combination antiparkinsonian agent. Levodopa is a dopamine precursor that crosses the blood-brain barrier and is converted to dopamine in the brain, replenishing depleted dopamine levels in the basal ganglia. Carbidopa is a peripheral dopa decarboxylase inhibitor that prevents the peripheral conversion of levodopa to dopamine, allowing more levodopa to reach the brain and reducing peripheral side effects like nausea and vomiting. This combination is the cornerstone of symptomatic treatment for Parkinson's disease.
Adult: Initial: 1 tablet (100mg/25mg) three times daily. Titrate gradually based on response and tolerance. Usual maintenance: 3 to 8 tablets per day in divided doses (typically 4-6 times daily). Doses > 8 tablets/day are rarely more effective.
Note: Take on an empty stomach, 30-60 minutes before or 1-2 hours after meals, with a full glass of water to maximize absorption. If nausea occurs, may take with a small, low-protein snack (e.g., crackers). Do not crush or chew sustained-release formulations. Swallow whole.
Levodopa, the metabolic precursor of dopamine, crosses the blood-brain barrier via the large neutral amino acid (LNAA) transporter. In the nigrostriatal neurons, it is decarboxylated by aromatic L-amino acid decarboxylase (AADC) to form dopamine, replenishing the depleted striatal dopamine levels characteristic of Parkinson's disease. Carbidopa, which does not cross the BBB, inhibits peripheral AADC. This inhibition reduces the extracerebral conversion of levodopa to dopamine, minimizing peripheral side effects (nausea, cardiovascular) and allowing approximately 4-5 times more levodopa to reach the brain.
Pregnancy: Category C (US FDA). Animal studies show adverse effects. Use only if potential benefit justifies potential risk to the fetus. Data in human pregnancy is limited.
Driving: May cause dizziness, syncope, and sudden episodes of somnolence. Patients should be cautioned against driving or operating machinery until their response is known.
| Non-selective MAO Inhibitors (e.g., Phenelzine, Tranylcypromine) | Risk of hypertensive crisis, hyperpyrexia. | Contraindicated |
| Antipsychotics (Typical: Haloperidol, Chlorpromazine; Atypical: Risperidone) | Antagonize dopaminergic effect, worsening parkinsonism. | Major |
| Antihypertensives | Additive hypotensive effect. | Moderate |
| Anticholinergics (e.g., Trihexyphenidyl) | May enhance efficacy but also side effects like confusion. | Moderate |
| Ferrous Sulfate (Iron) | Chelation reduces levodopa absorption and bioavailability. | Moderate |
| Protein-rich foods / Dietary supplements | Competes for absorption via LNAA transporter, reduces efficacy. | Moderate |
| Dopamine D2 receptor antagonists (Metoclopramide) | Worsens parkinsonian symptoms. | Major |
| Selective MAO-B Inhibitors (Selegiline, Rasagiline) | Enhanced dopaminergic effect; may increase dyskinesias. Dose adjustment may be needed. | Moderate |