What the FDA labels say about taking them together
Folic acid and isoniazid
Folic acid's FDA label (Folic Acid) of February 11, 2026 names isoniazid in its drug interactions section; the isoniazid label PlainMeds reads does not name folic acid. The folic acid label: “Valproate and sulfasalazine primarily reduce intestinal folate absorption or interfere with folate-dependent metabolic pathways, while isoniazid and cycloserine reduce folate utilization through metabolic disruption.”
Check it at the source: the folic acid FDA label on DailyMed (effective July 24, 2026, Rising Pharma Holdings, Inc.); the isoniazid FDA label on DailyMed (effective July 8, 2025, OMNIVIUM PHARMACEUTICALS LLC.).
What does the folic acid label say about isoniazid?
Folic Acid label (Ayurax, LLC, February 11, 2026), drug interactionsValproate and sulfasalazine primarily reduce intestinal folate absorption or interfere with folate-dependent metabolic pathways, while isoniazid and cycloserine reduce folate utilization through metabolic disruption.
Source: FDA drug label (Folic Acid, NDA216395), Ayurax, LLC, effective February 11, 2026 (SPL set 96618bf8-6c03-4df3-9896-b7ef27ca392a), via openFDA
Source: FDA drug label (Folic Acid), Rising Pharma Holdings, Inc., effective July 24, 2026 (SPL set 0baa10dd-6d1d-4946-8236-566451132da3), via openFDA; the full label on DailyMed
What does the isoniazid label say about folic acid?
Isoniazid's label (effective July 8, 2025) does not name folic acid 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
| Folic acid | Isoniazid | |
|---|---|---|
| Labeled for | n/a | n/a |
| Boxed warning | No | Yes |
| Pharmacy cost (NADAC) | $0.0201 per tablet 1 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