Are there any long-term side effects of being on ADHD meds?

None known. A prospective study is a research project that looks ahead with 2 groups treated/controls. Then sees the outcomes for many years ahead. There is an ongoing adhd prospective study run by the psychopharmacology dept at the mass general hosp at harvard. It started in 1990, and continues to this day. Periodically the authors publish reports. So far no problems have emerged in the adhd treated people.
Somes. There now are several fairly longterm studies of treating adhd. Longterm use of stimulants may have an effect on weight (lower at least for the first few years compared to unmedicated children with adhd) and height (slower growth rate, especially the first year or so). The two are not related. Look for publications from mta. No effect on heart rate or blood pressure.
No Significant diff. In short, nothing that has been definitive in studies. There are dozens of studies that have looked into this and while some indicated concerns for things like depression and other pysch disorders, other studies show no link. There is a small increase in sleep problems and some kids have trouble gaining weight. No other significant changes have been shown.