Phenytoin has been associated with reduced
lamotrigine serum levels (by almost two- thirds in one study).
Lamotrigine has no effect on
phenytoin levels.
Phenytoin (and therefore probably fosphenytoin) induces the
metabolism of
lamotrigine, and the recommended starting dose and long-term maintenance dose of
lamotrigine in patients already taking
phenytoin is twice that of patients receiving
lamotrigine monotherapy. However, note that if they are also taking valproate in addition to
phenytoin, the
lamotrigine dose should be reduced.