With ADHD, I see pretty marked differences in some (not all) students on/off psychostimulants, both on cognitive and achievement testing. Because of its impact on attention and persistence, anxiety meds can also affect performance. While working memory and processing speed (including fluency measures) tend to be the most sensitive to medication, other areas I notice (anecdotally) effects include visual organization and extended writing.
BTW, some psychotropics have psychomotor retarding/blunting effects, especially some mood stabilizers and antipsychotics, so you may see lower scores on meds, too.
