What the FDA labels say about taking them together
Carbidopa and isoniazid
Carbidopa's FDA label of July 2, 2020 names isoniazid in its drug interactions section; the isoniazid label PlainMeds reads does not name carbidopa. The carbidopa label: “Dopamine D 2 receptor antagonists and isoniazid may reduce the therapeutic effects of levodopa.”
Check it at the source: the carbidopa FDA label on DailyMed (effective July 2, 2020, Oceanside Pharmaceuticals); the isoniazid FDA label on DailyMed (effective July 8, 2025, OMNIVIUM PHARMACEUTICALS LLC.).
What does the carbidopa label say about isoniazid?
Carbidopa label, drug interactionsDopamine D 2 receptor antagonists (e.g., phenothiazines, butyrophenones, risperidone) and isoniazid may reduce the therapeutic effects of levodopa.
Source: FDA drug label (Carbidopa), NDA017830 (Aton), as published by Oceanside Pharmaceuticals, effective July 2, 2020 (SPL set d5204e5e-10ef-4758-8fc6-cb710048c687), via openFDA; the full label on DailyMed
What does the isoniazid label say about carbidopa?
Isoniazid's label (effective July 8, 2025) does not name carbidopa in these sections. A label lists what its studies and reports found; it is not a complete list.
Source: FDA drug label (isoniazid), OMNIVIUM PHARMACEUTICALS LLC., effective July 8, 2025 (SPL set 35400f1d-fd25-4bfa-a382-ac390e21bf2e), via openFDA; the full label on DailyMed
These are the labels' own sentences. PlainMeds does not grade interactions; a pharmacist or prescriber can say what they mean for one person's medicines.
Both medicines on the record
| Carbidopa | Isoniazid | |
|---|---|---|
| Labeled for | Parkinson's disease | n/a |
| Boxed warning | No | Yes |
| Pharmacy cost (NADAC) | $0.536 per tablet 25 mg tablet | $3.40 per tablet 300 mg tablet |
| Shortage now | No current entry | No current entry |
Source: FDA NDC Directory, CMS NADAC of October 7, 2026 and the FDA Drug Shortage Database (FDA files October 8, 2026)
What next?
- Check more medicines together
- up to five at once