By intrapatient comparison atconstant phenytoin (PUT) dose, the effect of increased carbamazepine (CBZ) dose was studied in 32 epileptic outpatients treated with a combination of PHT and CBZ. The mean PHT plasma concentration, as well as the concentration/dose ratio for PHT, became significantly higher secondary to increased doses of CBZ (14.1 ± 3.5 vs. 19.3 ± 3.6 μg/ml and 2.8 ± 1.0 vs. 3.9 ± 1.4 μg/ml plasma per milligram/kilogram daily dose, respectively; p < 0.001). Concomitantly, in spite of CBZ dose higher by 17.6%, the CBZ concentration increased by only 6.4%, and the CBZ concentration/dose ratio actually decreased by 10%. In contrast, by intrapatient comparison atconstant CBZ dose, the effect of reduced PHT dose on CBZ was studied in 22 patients. The mean CBZ plasma concentration as well as the concentration/dose ratio for CBZ appeared significantly higher, with a concomitant reduction of PHT (6.7 ± 1.6 vs. 8.6 ± 1.6 μg/ml and 0.37 ± 0.1 vs. 0.49 ± 0.2 μg/ml plasma per milligram/kilogram daily dose, respectively; p < 0.001). This simultaneous dual effect—inhibition of PHT metabolism by CBZ and induction of CBZ metabolism by PHT—can result in PHT intoxication along with a fall in CBZ plasma concentration to a subtherapeutic range. This effect may be avoided or reduced if the PHT concentration is adjusted to ∼13 μg/ml before CBZ is added or increased.