Carbamazepine toxicity, attributed to use of terfenadine, hasbeen described in one case report but interaction is not established.
Clinical evidence,mechanism, importance and management
A 18-year-old woman taking carbamazepine after treatment for brain metastases,developed confusion, disorientation, visual hallucinations, nausea and ataxia shortly after starting terfenadine 60mg twice daily for rhinitis. The symptoms were interpreted as carbamazepine toxicity. However,her total carbamazepine serum levels of 8.9 mg/L were within normal range. An interaction due to protein binding displacement was suspected and measurement of free carbamazepine revealed levels of 6 mg/L, almost three times upper limit of normal. All symptoms disappeared when terfenadine was stopped. The authors speculate that terfenadine had displaced carbamazepine from its plasma protein binding sites, thereby increasing levels of free and active carbamazepine (See reference number 1). The report is very brief and does not say whether any other drugs were being taken concurrently,so that this interaction is not established.
1. Hirschfeld S,Jarosinski P. Drug interaction of terfenadine and carbamazepine. Ann Intern Med (1993) 118, 907–8.