1. Clinical Overview
A fixed-dose combination of the dopamine precursor Levodopa and the peripheral aromatic L-amino acid decarboxylase (AADC) inhibitor Carbidopa. It is the cornerstone of symptomatic treatment for Parkinson's disease (PD). Carbidopa prevents the peripheral conversion of Levodopa to dopamine, allowing a greater proportion of Levodopa to cross the blood-brain barrier and be converted to dopamine in the striatum, thereby enhancing central efficacy and reducing peripheral dopaminergic side effects like nausea and vomiting.
| Onset | Duration | Bioavailability |
|---|---|---|
| 30 to 60 minutes (oral) | Approximately 4 to 6 hours per dose, but diminishes with disease progression ('wearing-off' phenomenon). | Levodopa: ~30% (highly variable due to extensive first-pass metabolism and competition with dietary amino acids). Carbidopa bioavailability is not a primary clinical determinant. |
2. Mechanism of Action
Levodopa is a metabolic precursor to dopamine. In Parkinson's disease, dopaminergic neurons in the substantia nigra degenerate, leading to dopamine deficiency in the striatum. Levodopa crosses the blood-brain barrier via active transport, where it is decarboxylated by central aromatic L-amino acid decarboxylase (AADC) to form dopamine, replenishing depleted striatal dopamine levels. Carbidopa, which does not cross the BBB, inhibits peripheral AADC, reducing the conversion of Levodopa to dopamine outside the CNS. This increases the bioavailability of Levodopa to the brain (by 5-10 fold) and minimizes peripheral dopaminergic side effects, allowing for lower and more effective dosing.
3. Indications & Uses
- Idiopathic Parkinson's Disease
- Post-encephalitic Parkinsonism
- Symptomatic Parkinsonism (e.g., due to carbon monoxide or manganese intoxication)
4. Dosage & Administration
Adult Dosage: Initial: 1 tablet (100mg/10mg) three or four times daily. Titrate slowly based on response and tolerance. Usual maintenance: 4 to 8 tablets per day in divided doses (every 4-8 hours). Maximum daily dose is individual tolerance, but often limited by dyskinesias.
Administration: Take on an empty stomach, 30-60 minutes before meals or 1-2 hours after meals to enhance absorption. If nausea occurs, may take with a small, low-protein snack (e.g., crackers). Do not crush or chew sustained-release formulations (if applicable). Swallow whole with water.
5. Side Effects
Common side effects may include:
- Nausea, vomiting (reduced by Carbidopa but still common initially)
- Anorexia
- Dizziness, lightheadedness (orthostatic hypotension)
- Dry mouth
- Abnormal dreams
- Dark discoloration of saliva, urine, or sweat (harmless).
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Non-selective MAO Inhibitors (e.g., Phenelzine, Tranylcypromine) | Risk of severe hypertensive crisis, hyperpyrexia. | Contraindicated |
| Antipsychotics (Typical: Haloperidol; Atypical: Risperidone) | Antagonize dopaminergic effect, worsening Parkinsonism. | Major |
| Antihypertensives | Additive hypotensive effect, risk of severe orthostasis. | Moderate |
| Ferrous Sulfate (Iron) | Chelation of Levodopa, reducing its absorption and efficacy. | Moderate |
| Protein-rich foods / Dietary Amino Acids | Competes for GI absorption and BBB transport, reducing efficacy. | Moderate |
| Anticholinergics (e.g., Trihexyphenidyl) | Additive therapeutic effect but also additive side effects (confusion, dry mouth). | Moderate |
| Dopamine D2 receptor antagonists (Metoclopramide) | Worsens parkinsonian symptoms. | Major |
| Selective MAO-B Inhibitors (Selegiline, Rasagiline) | Potentiation of Levodopa effect; may require dose reduction of Levodopa to avoid dyskinesias. | Moderate |
7. Patient Counselling
- DO take doses on an empty stomach (30-60 min before or 1-2 hrs after meals).
- DO report any new skin lesions or changes in moles to your doctor immediately.
- DO rise slowly from sitting/lying position to avoid dizziness.
- DO keep a consistent daily schedule for medication and meals.
- DON'T suddenly stop taking this medicine.
- DON'T take with high-protein meals.
- DON'T take iron supplements within 2-3 hours of this medicine.
8. Toxicology & Storage
Overdose: Exaggeration of known adverse effects: severe nausea/vomiting, cardiac arrhythmias, hypotension, intense dyskinesias, confusion, agitation, hallucinations, psychosis. In extreme cases: coma, hyperpyrexia.
Storage: Store below 30°C. Protect from light and moisture. Keep in the original blister pack or container. Keep out of reach of children.