The results revealed no evidence of difference between the two drugs. The potential for pitfalls when the neuropsychological effects of two AEDs are compared is illustrated by the reports of Dodrill and Troupin (33,34). In their original crossover study of PHT and CBZ (33), the performance of adult patients was significantly better for five test variables while they were taking CBZ. However, during the two study periods, the patients’ mean drug levels were 30.6 mg/L for PHT and 9.3 mg/L for CBZ. Since the PHT levels were disproportionately higher by today’s standards, the data were later reanalyzed (34). When patients with baseline PHT levels of 240 mg/L were eliminated from the analysis, only two test variables favored CBZ, and when patients with baseline PHT levels of >30 mg/L were eliminated, there were no statistically significant differences. This reanalysis does demonstrate the shortcomings of the initial study, but it is not in itself a valid demonstration of a lack of difference between the two drugs, the reason being that a high percentage of patients had to be excluded from the PHT arm and the original randomization was thus invalidated. In a more recent comparison between PHT and CBZ in adults, a randomized, double blind, double crossover design was used (35). Both drugs impaired performance on five cog- nitive tests. Comparison between the two drugs, using blood levels as covariates, revealed only two significant differences, one in favor of PHT and one in favor of CBZ. Sequential comparison of the effect of AEDs may also at times lead to questionable conclusions. For in- stance, VPA has been reported to have a psychotropic effect (36), possibly based on discontinuation of previous medications. In a controlled study, VPA was found to have no psychotropic effect and to negatively affect motor speed and visuospatial functions (37).