What has been said so far is accurate but it is not a 'harmless' drug
One area that needs further study regarding the use of Ritalin and ADHD/hperactivity is that although it enables the student to focus for longer periods of time (which helps the student demonstrate what has already been learned) there is increasing evidence that very little 'new learning' occurs when using Ritalin.
In other words, it is very difficult to teach new ideas to a student who is using Ritalin.
The other problem with Ritalin is that it is over prescribed and although there is supposed to be a trial period and data, this is rarely accomplished. Very little observation, if any, is undertaken during the trial period and most of it is highly subjective.