Just an FYI, My son who is 16 is on 25 mg of Adderall. He only takes it on school days. Not on weekends and not during the summer. He doesn't miss it on the days he doesn't take it. All he says to me is that it does help him focus when he does take it.
In the book, "The ADD/ ADHD Answer book," by Susan Ashley, she states (p162) "providing the medication is taken as prescribed, there is not a risk for addiction. Dosages are low and not anywhere near the amount required to develop a dependency." She also states that (p163), "studies have consistently found no association between stimulant medication and increased risk of later drug use." In fact, "stimulant medication reduced the risk for substance abuse by about half in adolescents."
I think it would be worth your time to get this book. Its quite cheap on Amazon and it has a lot of information that I think you need since obviously your doctor has bothered to do so.