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Carbamazepine and oxcarbazepine are structurally related to TCAs. Like fosphenytoin, oxcarbazepine is a prodrug that is metabolized to an active metabolite. Carbamazepine and oxcarbazepine are anticonvulsants.
- The therapeutic efficacy of carbamazepine and oxcarbazepine is due to sodium channel blockade, which prevents the propagation of an abnormal focus. The therapeutic serum concentration of carbamazepine is 4–12 mg/L. There is no routine laboratory testing for oxcarbazepine; however, the carbamazepine assay will detect the presence of oxcarbazepine.
- The toxicity associated with carbamazepine is likely due to its chemical structure. TCA-like effects include sodium channel blockade, QT prolongation, and anticholinergic features.
- In overdose, carbamazepine is erratically absorbed and may form concretions in the GI tract causing prolonged toxicity.
- Persistently high levels of carbamazepine have been reported to increase antidiuretic hormone secretion leading to syndrome of inappropriate antidiuretic hormone release (SIADH).1
Carbamazepine toxicity may be enhanced by concomitant use of drugs that are metabolized by the cytochrome P450 system.