What the FDA labels say about taking them together
Methsuximide and phenytoin
Methsuximide's FDA label of April 7, 2026 names phenytoin in its drug interactions section, and phenytoin's label names methsuximide too. The methsuximide label: “Drug Interactions Since Celontin (methsuximide) may interact with concurrently administered antiepileptic drugs, periodic serum level determinations of these drugs may be necessary.”
Check it at the source: the methsuximide FDA label on DailyMed (effective April 7, 2026, Parke-Davis Div of Pfizer Inc); the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.).
What does the methsuximide label say about phenytoin?
Methsuximide label, drug interactionsDrug Interactions Since Celontin (methsuximide) may interact with concurrently administered antiepileptic drugs, periodic serum level determinations of these drugs may be necessary (eg, methsuximide may increase the plasma concentrations of phenytoin and phenobarbital).
Methsuximide label, warningsData are more extensive with respect to phenytoin and phenobarbital, but these are also the most commonly prescribed anticonvulsants; less systematic or anecdotal reports suggest a possible similar association with the use of all known anticonvulsant drugs.
Source: FDA drug label (Celontin), Parke-Davis Div of Pfizer Inc, effective April 7, 2026 (SPL set 64a6ee88-c6b1-4e13-8208-b6772ef65a74), via openFDA; the full label on DailyMed
What does the phenytoin label say about methsuximide?
Phenytoin label, drug interactionsInteracting Agent: Antiepileptic drugs; Examples: Ethosuximide, felbamate, oxcarbazepine, methsuximide, topiramate
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
| Methsuximide | Phenytoin | |
|---|---|---|
| Labeled for | Seizures and epilepsy | Seizures and epilepsy |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | $4.40 per capsule 300 mg capsule | $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