What the FDA labels say about taking them together
Fosphenytoin and methadone
Fosphenytoin's FDA label of May 18, 2025 names methadone in its drug interactions section; the methadone labels PlainMeds reads (4) do not name fosphenytoin. The fosphenytoin label: “Interacting Agent: Other; Examples: Albendazole, chlorpropamide, clozapine, cyclosporine, digoxin, disopyramide, folic acid, methadone, mexiletine, praziquantel, quetiapine”
Check it at the source: the fosphenytoin FDA label on DailyMed (effective May 18, 2025, Pfizer Laboratories Div Pfizer Inc); the methadone FDA label on DailyMed (effective December 8, 2025, Epic Pharma, LLC).
What does the fosphenytoin label say about methadone?
Fosphenytoin label, drug interactionsInteracting Agent: Other; Examples: Albendazole (decreases active metabolite), chlorpropamide, clozapine, cyclosporine, digoxin, disopyramide, folic acid, methadone, mexiletine, praziquantel, quetiapine
Source: FDA drug label (CEREBYX), NDA020450 (Parke Davis), as published by Pfizer Laboratories Div Pfizer Inc, effective May 18, 2025 (SPL set de5a6711-4abb-4c3d-a18b-266b094ace11), via openFDA; the full label on DailyMed
What does the methadone label say about fosphenytoin?
Methadone's label (effective December 8, 2025) and its 3 other brand-application labels do not name fosphenytoin in these sections. A label lists what its studies and reports found; it is not a complete list.
Source: FDA drug label (Methadone Hydrochloride), Epic Pharma, LLC, effective December 8, 2025 (SPL set 6da7501b-b881-4bc7-891a-74214a00a3ac), 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
| Fosphenytoin | Methadone | |
|---|---|---|
| Labeled for | Seizures and epilepsy | Pain, Opioid use disorder |
| Boxed warning | Yes | Yes |
| Pharmacy cost (NADAC) | n/a | $0.177 per tablet 10 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