Phenobarbital has been associated with reduced
lamotrigine serum levels (reduction of about 50% in one study).
Lamotrigine does not appear to affect
phenobarbital or
primidone levels.
Phenobarbital induces the
metabolism of
lamotrigine, and the recommended starting dose and long-term maintenance dose of
lamotrigine in patients already taking
phenobarbital or
primidone is twice that of patients receiving
lamotrigine monotherapy. However, note that if they are also taking valproate in addition to
phenobarbital, the
lamotrigine dose should be reduced.