Methamphetamine has a bad rep, but taken orally in small quantities it is not much more dangerous than adderall, and has pretty much the same side effects. It can be legally prescribed for ADHD (brand name Desoxyn).
Now, if the difference between a “left-handed” and “right-handed” meth molecule can be so great, it shouldn’t come as a surprise that adding a methyl group to regular amphetamine will change its effects considerably.
This can be seen in the different receptor binding profiles (which receptors, where in the brain, and how strong or long the modulation is).
Meth’s increased addictive and neurotoxic effects are owed to this difference in receptor binding.