This large, long study found that children had worse academic outcomes after being treated with Ritalin, a common medication used in the treatment of ADHD. A 1997 policy reform in Quebec expanded coverage and use of Ritalin, providing ideal conditions to study its use relative to the rest of Canada. Generally, there were little overall improvements in short term outcomes, and worsened long term outcomes, highlighted by increased incidents of repeating grades, lower standardized math scores, and more school dropouts.
One especially interesting consequence of increased ritalin use was a large reported increase in unhappiness, especially among girls. The study authors hypothesized that increased Ritalin use, while decreasing adverse behaviors, also decreased attention these students received from teachers. They surmised that use of these medications may be a substitute for more beneficial learning interventions.
A study summary from The Atlantic can be found here: http://www.theatlantic.com/health/archive/2013/06/study-ritalin-doesnt-help-academics/276894/
A link to the full study can be found here: http://www.nber.org/papers/w19105.pdf