You're confusing methylphenidate (active ingredient in Ritalin/Concerta) and amphetamine salts which are used in the other family of stimulants such as Adderall, Vyvanse, etc...

Methamphetamine(as desoxym) is also available as a prescription for add/ADHD, though like the OP I've never heard any cases of it actually being used, and i believe it's pretty rare. http://www.rxlist.com/script/main/mobileart-rx.asp?drug=deso...

Desoxyn is literally brand-name prescription methamphetamine.