What the FDA labels say about taking them together
Ethosuximide and phenytoin
Ethosuximide's FDA label of June 11, 2026 names phenytoin in its warnings section, and phenytoin's label names ethosuximide too. The ethosuximide label: “Drug Interactions: Since Zarontin (ethosuximide) may interact with concurrently administered antiepileptic drugs, periodic serum level determinations of these drugs may be necessary.”
Check it at the source: the ethosuximide FDA label on DailyMed (effective June 11, 2026, Chartwell Governmental & Specialty RX, LLC.); the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.).
What does the ethosuximide label say about phenytoin?
Ethosuximide 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.
Zarontin label (Parke-Davis Div of Pfizer Inc, May 25, 2026), drug interactionsDrug Interactions: Since Zarontin (ethosuximide) may interact with concurrently administered antiepileptic drugs, periodic serum level determinations of these drugs may be necessary (e.g., ethosuximide may elevate phenytoin serum levels and valproic acid has been reported to both increase and decrease ethosuximide levels).
Source: FDA drug label (Zarontin, NDA012380), Parke-Davis Div of Pfizer Inc, effective May 25, 2026 (SPL set 0e008f33-70a1-4bc6-b3a0-d45214418ab6), via openFDA
Source: FDA drug label (Ethosuximide), Chartwell Governmental & Specialty RX, LLC., effective June 11, 2026 (SPL set a825265d-ace9-4463-a16d-f6d5e1e31188), via openFDA; the full label on DailyMed
What does the phenytoin label say about ethosuximide?
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
| Ethosuximide | Phenytoin | |
|---|---|---|
| Labeled for | Seizures and epilepsy | Seizures and epilepsy |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | $0.243 per capsule 250 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