What the FDA labels say about taking them together
Nitisinone and phenytoin
Nitisinone's FDA label of November 30, 2021 names phenytoin in its drug interactions section; the phenytoin labels PlainMeds reads (2) do not name nitisinone. The nitisinone label: “Clinically Relevant Interactions Affecting Co-Administered Drugs Sensitive CYP2C9 Substrates or CYP2C9 Substrates with a Narrow Therapeutic Index Clinical Impact Increased exposure of the co-administered drugs …”
Check it at the source: the nitisinone FDA label on DailyMed (effective November 30, 2021, SWEDISH ORPHAN BIOVITRUM AB (PUBL)); the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.).
What does the nitisinone label say about phenytoin?
Nitisinone label, drug interactionsClinically Relevant Interactions Affecting Co-Administered Drugs Sensitive CYP2C9 Substrates (e.g., celecoxib, tolbutamide) or CYP2C9 Substrates with a Narrow Therapeutic Index (e.g., phenytoin, warfarin) Clinical Impact Increased exposure of the co-administered drugs metabolized by CYP2C9. [see Clinical Pharmacology ( 12.3 )] Intervention Reduce the dosage of the co-administered drugs metabolized by CYP2C9 drug by half. Additional dosage adjustments may be needed to maintain therapeutic drug concentrations for narrow therapeutic index drugs. See prescribing information for those drugs.
Source: FDA drug label (Nityr, NDA209449), Cycle Pharmaceuticals Ltd., effective December 22, 2025 (SPL set 00fd1905-27e4-420e-8dc5-a69e4ddc1526), via openFDA
Source: FDA drug label (Orfadin), SWEDISH ORPHAN BIOVITRUM AB (PUBL), effective November 30, 2021 (SPL set 5d449b73-d503-4132-b978-d890491975df), via openFDA; the full label on DailyMed
What does the phenytoin label say about nitisinone?
Phenytoin's label (effective October 15, 2024) and its 1 other brand-application label do not name nitisinone 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
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
| Nitisinone | Phenytoin | |
|---|---|---|
| Labeled for | n/a | Seizures and epilepsy |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | n/a | $0.145 per capsule 100 mg capsule |
| 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