A patient who is being treated for temporal lobe epilepsy (complex paial seizures) and who is having recurrent seizures on his chronic regimen of carbamazepine is given phenobarbital as a second drug. However, the seizures increase in frequency. What is the probable reason for the apparent deleterious effect of adding phenobarbital:
Intracerebral bleeding from worsening bone marrow suppression
Decreased carbamazepine level
Decreased stability of CNS neuronal membranes
Hypokalemia
Answer is B (Decreased Carbamazepine levels): Phenobarbitone is a microsomal enzyme inducer so administration of phenobarbitone induces the metabolism of carbamazepine and reduces its level in blood.