Why can't dopamine cross the blood brain barrier but l-dopa can? Levodopa can cross the BBB and is less lipid soluble then dopamine, so dopamine should be able to cross the BBB more easily then l-dopa, but for some reason dopamine cant cross the BBB. Both dl-methamphetamine and levomethamphetamine can cross the blood brain barrier, but why not dl-dopamine?

Both L-Dopa and Dopamine are polar molecules and thus hydrophilic. They are solvable in water but not in lipids, so they are not able to cross the Blood-brain barrier on their own.

The difference is that L-Dopa has a transport protein (SLC7A5) that allows it to pass through the Blood-brain barrier, whereas dopamine doesn't.