Some reports describe rises in
serum phenytoin levels, with
toxicity, whereas others describe falls in
phenytoin levels. Genetic differences in the
metabolism of these drugs may be an explanation for the differences. Falls in
carbamazepine serum levels, sometimes with rises in carbamazepine-epoxide levels, have been described.
Monitor antiepileptic levels during concurrent use (where possible including carbamazepine-epoxide, the active
metabolite of carbamazepine) so that steps can be taken to avoid the development of
toxicity or lack of
efficacy. Not all patients appear to have an adverse interaction, and, at present, it does not seem possible to identify those potentially at risk.