Lamotrigine and Methsuximide: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Lamotrigine
Methsuximide
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Reduced lamotrigine concentrations and possible loss of seizure control
Interaction Deep Dive
During a retrospective study, it was determined that methsuximide significantly decreased lamotrigine plasma concentrations. In 13 patients with epilepsy receiving combination therapy, lamotrigine concentrations were 69.7% lower than compared to lamotrigine monotherapy, indicating that methsuximide strongly induces lamotrigine metabolism1.
Why it happens (mechanism)
Hepatic induction by methsuximide of lamotrigine metabolism
Literature reports
1 report — tap to read
a) Lamotrigine serum concentrations from 222 patients receiving lamotrigine monotherapy (n = 64) or combination therapy with another antiepileptic agent were evaluated. Thirteen patients were being treated with lamotrigine and methsuximide. In the lamotrigine monotherapy group, the lamotrigine concentration was 7.14 mcg/mL (27.88 mcmol/L) while the mean dose was 7.27 mg/dose/kg. The lamotrigine level-to-dose ratio (LDR) in this group calculated out to 1.07 mcg/mL/mg/kg. In the subjects receiving methsuximide in addition to lamotrigine, the lamotrigine plasma concentration was 3.06 mcg/mL (11.95 mcmol/L) while the mean dose was 9.55 mg/dose/kg. The lamotrigine LDR in this group was 0.31 mcg/mL/mg/kg, demonstrating the inducing properties of methsuximide on lamotrigine metabolism 1.
Common questions
Can I take Lamotrigine and Methsuximide together?
Reduced lamotrigine concentrations and possible loss of seizure control Always confirm with your pharmacist or prescriber before making any change.
How serious is the Lamotrigine and Methsuximide interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "delayed". Effects tend to build up gradually over days to weeks.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Lamotrigine or Methsuximide need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there anything you'd monitor while I'm on both?
References (1)
- May TW, Rambeck B, & Jurgens U: Influence of oxcarbazepine and methsuximide on lamotrigine concentrations in epileptic patients with and without valproic acid comedication: results of a retrospective study. Ther Drug Monit 1999; 21:175-181. PubMed
Keep reading about Lamotrigine
Keep reading about Methsuximide
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