Probably the only guarantee is that the meds have been used for a very long time. Ritalin has been used since at least the early '70's, and Adderall since 1996. Perhaps that is why Susan Ashley in her book ADD & ADHD AnswerBook on page 164 says that, "With the millions of children treated, there are no known permanently damaging effects. To date there have been no known studies suggesting the presence of negative long-term effects". But I also doubt that much money has been made available to have done the long term studies.
There is a very good site where you can get a lot of information on the various treatments and outcomes - its -
I (only personal opinion here) think that if you are using them only on a as needed basis, you should be ok. I do think that it would be wise to explore all of the study techniques out there to enhance your productivity. Also since you must be already diagnosed with ADHD or ADD, I would make sure you have an IEP or a 504. It will help you not only in high school, but also in college. Best wishes!