Factors influencing plasma phenobarbitone levels in epileptic patients.

1 Various statistical techniques were used to study the effects of age, sex and concurrent therapy with other anticonvulsants on the relation between plasma phenobarbitone levels and doses of (i) phenobarbtione, (ii) methylphenobarbitone or (iii) primidone, in epileptic patients. 2 Methylphenobarbitone and primidone are converted to phenobarbitone in the body. The mean doses of phenobarbitone, methylphenobarbitone and primidone which produced the same plasma phenobarbitone level (15 microgram/ml) were, respectively, 1.75,2.75 and 7.75 mg kg-1 day-1. 3 For both phenobarbitone and methylphenobarbitone dose requirement to achieve a given plasma phenobarbitone level fell progressively with age. Sex influenced the relation between plasma phenobarbitone level and phenobarbitone or methylphenobarbitone dose. Interactions were detected between primidone and both phenytoin and carbamazepine. 4 In individual patients, within the limits of dosage studied, the relation between plasma phenobarbitone level and drug dose was not rectilinear if phenobarbitone itself was taken, but was rectilinear if methylphenobarbitone was taken.