What the FDA labels say about taking them together
Methsuximide and phenobarbital
Methsuximide's FDA label of April 7, 2026 names phenobarbital in its drug interactions section; phenobarbital's label does not name methsuximide itself but warns about its class (Anti-seizure medicines). 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 phenobarbital FDA label on DailyMed (effective August 27, 2026).
What does the methsuximide label say about phenobarbital?
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 phenobarbital label say about methsuximide?
Phenobarbital's label does not name methsuximide itself; it warns about its class (Anti-seizure medicines):
Phenobarbital label, warnings and precautionsIf a rash occurs, the patient should be evaluated for signs and symptoms of Drug Reaction with Eosinophilia and Systemic Symptoms [see Warnings and Precautions (5.6)]. Reaction with Eosinophilia and Systemic Symptoms (DRESS)/Multiorgan Hypersensitivity Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS), also known as multiorgan hypersensitivity, has been reported in patients taking antiepileptic drugs, including phenobarbital.
Phenobarbital label, warnings and precautionsAntiepileptic drugs (AEDs) increase the risk of suicidal thoughts or behavior in adolescents and adults.
Source: FDA drug label (Phenobarbital), Chartwell RX, LLC, effective August 27, 2026 (SPL set 74428144-fb24-40a5-a670-3b876e2d116c), 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 | Phenobarbital | |
|---|---|---|
| Labeled for | Seizures and epilepsy | Seizures and epilepsy |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | $4.40 per capsule 300 mg capsule | $0.213 per tablet 100 mg tablet |
| Shortage now | No current entry | A presentation being discontinued |
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