This study found that ritalin use leads to cocaine abuse.

Stimulant treatment in high school also did not influence drug use in
adulthood except for greater use of cocaine.
http://pediatrics.aappublications.or...tract/111/1/97

They found, unfortunately, that treatment with stimulant medication in high
school is significantly associated with the risk of ever trying cocaine.
That this is a valid conclusion is supported by their finding that treatment
with stimulant medication in childhood is associated with cocaine use in
adulthood, although after adjusting for "disease severity" and the presence
of conduct disorder, the association was no longer significant.
The authors work extraordinarily hard to explain to the reader why the first
finding, which is statistically significant, is actually not real. The naïve
reader may wonder why the authors are so invested in undermining their own
carefully performed results.

http://pediatrics.aappublications.or...act/112/6/1459



It is not hard to explain. The study author is a big promoter of stimulant
medication
for ADHD, and has written a book on the subject.
http://www.amazon.com/exec/obidos/tg...234100-4292065