Prohibition or coffee shops: regulation of amphetamine and methylphenidate for enhancement use by healthy adults.

Eh. It's not that simple. Dopamine is an inherently neurotoxic neurotransmitter, but it's safely transported via the vesicular monoamine transporter 2 (VMAT2). Amphetamine causes a release of dopamine, which is normally okay, but it also a substrate for VMAT2- so when it gets picked up by VMAT2, instead of picking it up and moving on, it releases dopamine back into the cytosol which then interacts with the dopamine transporter, somehow causing it to induce reverse transport. Obviously VMAT2 doesn't exclusively pick up amphetamine, so it still clears up the dopamine, but it takes a longer time since the process is being interfered with. In the meantime, dopamine can work unchecked to induce oxidative stress.

Amphetamines do this to some degree, but I believe methamphetamine does it more actively. The problem is most of the research on VMAT is recent and it hasn't evolved into a clear picture yet. Furthermore, it doesn't take specific quantities of dopamine to cause any amount of damage, and oxidative stress is a relatively normal phenomenon in the day-to-day workings of the brain. The $100m question is how much is too much, and what amount of oxidative stress is severely damaging.