1. Clinical Overview
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, thereby increasing the amount of levodopa available to cross the blood-brain barrier and reducing peripheral side effects like nausea and vomiting. This combination is the cornerstone of symptomatic treatment for Parkinson's disease.
| Onset | Duration | Bioavailability |
|---|---|---|
| 30 to 60 minutes (oral) | Approximately 5 to 8 hours (dose-dependent) | Levodopa: ~30% (highly variable, reduced by food). Carbidopa: 40-70%. |
2. Mechanism of Action
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 to dopamine by aromatic L-amino acid decarboxylase (AADC), replenishing the depleted dopamine stores characteristic of Parkinson's disease. Carbidopa, an inhibitor of peripheral AADC, does not cross the blood-brain barrier. It inhibits the conversion of levodopa to dopamine in peripheral tissues, increasing the bioavailability of levodopa to the CNS by 5-10 fold and markedly reducing peripheral dopamine-induced adverse effects like nausea, vomiting, and cardiovascular effects.
3. Indications & Uses
- Idiopathic Parkinson's Disease
- Post-encephalitic Parkinsonism
4. Dosage & Administration
Adult Dosage: Initial: 1 tablet (250/25 mg) three times daily. Titrate slowly based on response and tolerance. Usual maintenance: 3 to 8 tablets daily in divided doses (typically 3-4 times daily). Doses above 8 tablets daily are rarely needed and require specialist supervision.
Administration: Take on an empty stomach, at least 30-45 minutes before 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
- Anorexia
- Dizziness
- Orthostatic Hypotension
- 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, Chlorpromazine; Atypical: Risperidone, Olanzapine) | Antagonize dopaminergic effect, worsening parkinsonism. | Major |
| Antihypertensives | Additive hypotensive effect. | Moderate |
| Ferrous Sulfate (Iron) | Forms chelates with levodopa, reducing its absorption and efficacy. | Moderate |
| Metoclopramide | Antagonizes dopaminergic effect in CNS. | Major |
| Anticholinergics (e.g., Trihexyphenidyl) | May improve tremor but can worsen hallucinations/delirium and reduce gastric motility affecting levodopa absorption. | Moderate |
| COMT Inhibitors (e.g., Entacapone) | Increases bioavailability and duration of action of levodopa; requires dose reduction of levodopa to avoid dyskinesias. | Major |
| Dopamine D2 receptor antagonists (e.g., Metoclopramide) | Decreased efficacy of levodopa. | Major |
| Pyridoxine (Vitamin B6) | Reverses the peripheral decarboxylase inhibitory effect of carbidopa, increasing peripheral conversion and reducing central efficacy. Not a concern with adequate carbidopa dose (as in this combination). | Minor with this FDC |
7. Patient Counselling
- DO take medication on an empty stomach (30-45 min before or 1-2 hours after food).
- DO report any new skin lesions or changes in moles to your doctor immediately.
- DO inform all doctors and surgeons you are taking this medication.
- DO NOT stop taking this medicine suddenly. Sudden withdrawal can lead to Neuroleptic Malignant Syndrome (NMS).
- DO NOT take with a high-protein meal. Distribute protein intake evenly.
- DO NOT take with non-prescription cold or flu remedies without consulting your doctor/pharmacist.
8. Toxicology & Storage
Overdose: Exaggeration of adverse effects: Severe nausea/vomiting, cardiac arrhythmias, hypotension or hypertension, intense dyskinesias, confusion, agitation, hallucinations, psychosis. In extreme cases: Coma.
Storage: Store below 30°C. Protect from light and moisture. Keep in the original blister pack or container. Keep out of reach of children.