The difference in the brain for Adderall v. Vyvanse comes down to the concentration of which enantiomoers are (how much of which mirror image of amphetamine the

The difference in the brain for Adderall v. Vyvanse comes down to the concentration of which enantiomoers are (how much of which mirror image of amphetamine there is). It's all amphetamine, it all affects the same receptors. The real difference comes down to how long it stays available. Vyvanse sticks around in the blood longer than "plain" amphetamine, but amphetamine crosses into the brain much faster. Both of those are because the liver has to first cleave off the extra molecule tacked onto the amphetamine that makes it Vyvanse. That can only happen so fast, and until it does it's too big to cross the blood/brain barrier or get otherwise disposed of.