Carbamazepine (Tegretol) is a frequently prescribed anticonvulsant that is particularly effective against complex partial and generalized tonic-clonic seizures.1 Manifestations of carbamazepine toxicity include aplastic anemia, rashes, stomatitis, dizziness, diplopia, confusion, headache, anorexia, and nausea.2 Carbamazepine has been noted to have a variety of interactions with other medications. The use of carbamazepine in combination with oral anticoagulants, contraceptives, or doxycycline, decreased the effectiveness of these medications by the probable induction of microsomal enzymes by carbamazepine. When taken with propoxyphene or troleandomycin, an increased carbamazepine effect has been noted, probably due to inhibition of microsomal enzymes by those drugs.3 The following case report describes the interaction of carbamazepine with isoniazid.