The methyl group attached to the amine basically just helps it cross the blood brain barrier better and makes it more resistant to enzymatic destruction. This means it is a couple times more potent dose-per-dose and lasts longer. It's nothing like the difference between methanol and ethanol, where one will kill you and the other is fine to drink. That IMO a rationalization people with ADHD make to convince themselves their prescribed amphetamines are somehow qualitatively different than big scary meth--just like the rationalization that amphetamine acts magically different in people with ADHD (it calms everyone down in low doses, it just might not provide the same working-memory deficit correcting effects as there is no deficit). In fact, in most of Europe, amphetamine itself is what you find on the streets, not meth.