C. Lamotrigine is mainly metabolized via glucuronidation and does not have much effect on
other prescribed medications, but valproate has been observed to double the plasma levels of
lamotrigine. The elimination half-life of lamotrigine in healthy young adults is approximately 25 to
30 hours. In patients taking valproate this is prolonged to 60 hours. As a result, when lamotrigine
is initiated for patients who are taking valproate, the starting dose should be approximately
50% of the normal starting dose of lamotrigine. Some suggest that if valproate is started for a
patient who is already on lamotrigine, one must consider obtaining a baseline lamotrigine plasma
concentration. The lamotrigine dose may have to be reduced during the valproate dose titration.
As a result of this interaction, the incidence of lamotrigine-induced rash seems to be more
common in those taking the combination. Phenytoin, carbamazepine, and oral contraceptives may
reduce the half-life of lamotrigine.