What the FDA labels say about taking them together
Isoniazid and theophylline
Isoniazid's FDA label of July 8, 2025 names theophylline in its drug interactions section, and theophylline's label names isoniazid too. The isoniazid label: “Theophylline A recent study has shown that concomitant administration of isoniazid and theophylline may cause elevated plasma levels of theophylline, and in some instances a slight decrease in the elimination of …”
Check it at the source: the isoniazid FDA label on DailyMed (effective July 8, 2025, OMNIVIUM PHARMACEUTICALS LLC.); the theophylline FDA label on DailyMed (effective March 10, 2026, Strides Pharma Science Limited).
What does the isoniazid label say about theophylline?
Isoniazid label, drug interactionsTheophylline A recent study has shown that concomitant administration of isoniazid and theophylline may cause elevated plasma levels of theophylline, and in some instances a slight decrease in the elimination of isoniazid.
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
What does the theophylline label say about isoniazid?
Theophylline label, drug interactionsisoniazid; terbutaline, systemic
Source: FDA drug label (Theophylline), Strides Pharma Science Limited, effective March 10, 2026 (SPL set b7a90ccf-9c32-40cc-9008-7e55b3da4dfd), 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
| Isoniazid | Theophylline | |
|---|---|---|
| Labeled for | n/a | Asthma |
| Boxed warning | Yes | No |
| Pharmacy cost (NADAC) | $3.40 per tablet 300 mg tablet | $0.330 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