What the FDA labels say about taking them together
Phenytoin and theophylline
Phenytoin's FDA label of October 15, 2024 names theophylline in its drug interactions section, and theophylline's label names phenytoin too. The theophylline label: “Phenytoin Phenytoin increases theophylline clearance by increasing microsomal enzyme activity.”
Check it at the source: the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.); the theophylline FDA label on DailyMed (effective March 10, 2026, Strides Pharma Science Limited).
What does the phenytoin label say about theophylline?
Phenytoin label, drug interactionsInteracting Agent: Other; Examples: Chronic alcohol abuse, diazepam, diazoxide, folic acid, reserpine, rifampin, St. John’s wort The induction potency of St. John’s wort may vary widely based on preparation., sucralfate, theophylline
Source: FDA drug label (Phenytek), Mylan Pharmaceuticals Inc., effective October 15, 2024 (SPL set 0f94c791-8b07-4b41-927e-c9f28bec5f60), via openFDA; the full label on DailyMed
What does the theophylline label say about phenytoin?
Theophylline label, drug interactionsPhenytoin Phenytoin increases theophylline clearance by increasing microsomal enzyme activity.
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
| Phenytoin | Theophylline | |
|---|---|---|
| Labeled for | Seizures and epilepsy | Asthma |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | $0.145 per capsule 100 mg capsule | $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