What the FDA labels say about taking them together
Phenytoin and thiotepa
Thiotepa's FDA label of January 30, 2026 names phenytoin in its drug interactions section; the phenytoin labels PlainMeds reads (2) do not name thiotepa. The thiotepa label: “Avoid co-administration of strong CYP3A4 inhibitors and strong CYP3A4 inducers with TEPADINA due to the potential effects on efficacy and toxicity.”
Check it at the source: the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.); the thiotepa FDA label on DailyMed (effective January 30, 2026, Amneal Pharmaceuticals LLC).
What does the phenytoin label say about thiotepa?
Phenytoin's label (effective October 15, 2024) and its 1 other brand-application label do not name thiotepa in these sections. A label lists what its studies and reports found; it is not a complete list.
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 thiotepa label say about phenytoin?
Thiotepa label, drug interactionsAvoid co-administration of strong CYP3A4 inhibitors (e.g., itraconazole, clarithromycin, ritonavir) and strong CYP3A4 inducers (e.g., rifampin, phenytoin) with TEPADINA due to the potential effects on efficacy and toxicity [see Clinical Pharmacology ( 12.3 ) ].
Source: FDA drug label (TEPADINA), NDA208264 (Adienne Sa), as published by Amneal Pharmaceuticals LLC, effective January 30, 2026 (SPL set 559e353c-2dc0-16ba-e054-00144ff8d46c), 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 | Thiotepa | |
|---|---|---|
| Labeled for | Seizures and epilepsy | Organ transplant rejection |
| Boxed warning | No | Yes |
| Pharmacy cost (NADAC) | $0.145 per capsule 100 mg capsule | n/a |
| 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