Do high doses of stimulants impair flexible thinking in attention-deficit hyperactivity disorder?

To test the hypothesis that high doses of methylphenidate (MPH) impair cognitive flexibility in attention-deficit hyperactivity disorder (ADHD).

A double-blind crossover design was used in an acute dosage trial to assess effects of three dosages (0.3, 0.6, and 0.9 mg/kg) of MPH on the performance of 17 ADHD children on five tasks designed to assess divergent thinking, perseveration, and ability to shift mental set. The tasks also assessed convergent thinking, problem solving, and speed and accuracy of processing.

There was minimal evidence of deleterious effects on flexible thinking or other cognitive processes, either in the ADHD group as a whole or in any subgroup. The most common pattern indicated linear improvement across dosages.

Under the acute dosage conditions used in this study, MPH doses up to 0.9 mg/kg had an increasingly positive effect on measures of mental flexibility and other cognitive processes. Rather than eliciting perseveration, MPH appeared to improve persistence. The generalized nature of the cognitive and motivational changes observed suggests that MPH acts on central, self-regulatory processes. Because effects of two or more daily doses can accumulate when MPH is prescribed in the clinical situation, clinical doses of more than 0.6 mg/kg were not recommended.