No. I'm saying the opposite. Let's assign arbitrary numbers in terms of concentration disability, 100 being normal, <100 being less concentration.
Person 1: 100
Person 2: 90
Person 3: 40
Person 4: 20
Person 5: 0
With methylphenidate, and assuming that everything EXCEPT concentration ability (assuming due to ADHD) being held constant:
Person 1: 120
Person 2: 120
Person 3: 120
Person 4: 120
Person 5: 0 (no such person exists who would actually benefit from methylphenidate, since a person with no concentration ability is a vegetable
)
The drug works on the same pathways, and brings people to the same level, which is what the studies showed as best as possible. It would be extremely difficult to test whether or not it actually increases grades or not due to the massive amount of variables that would have to be accounted for to make the study anything other than garbage, but it is generally accepted by people who have a lot of experience with this drug and these disorders (...like me...) that this is the general trend with psychostimulants. Variations exist, but mainly due to body weight and other such factors.
But no, a person having ADD or ADHD would not make them more or less sensitive to the drug. Which is why, according to the logic of the opposition, ANYBODY who takes the drug, even with a prescription, is "cheating" because they too would have an "advantage."