From what I understand, methamphetamine is either always, or almost always, neurotoxic.
That said, in therapeutic doses, up to 25 mg per day per oral, studies are lacking (to say the least). Illegal venues tend to be relatively pure. Addicts may have 100 mg per day, but from what I know there exist studies on more like .5g and up. They also smoke/snort/shoot the stuff, making it more toxic...
Even so, 100 mg meth is ~200 mg amphetamine. That dose of ampehtamine is likely toxic, but add in the extra action(s) of meth, and all meth users almsot surely are putting their brain (and lesser but also significant, their heart) in atrophy mode. Also, staying up for a long time, not eating//drinking, a hot environment, and excessive exercise play into things.
MDMA is, imo, a serotonergic toxin unless in a clincial, therpauetic setting and dose (it tends to be impure, synergizing toxicity).
Methe is, imo, a more mild toxin, but to dopaminergic neurons. But also meth is wildly addictive. MDMA rarely becomes addictive, but even a few times rolling can cause serious issues.
Mehtylphenidate may be helppful to the brain in modest, therapeutic doses when indicated. No one is saying amphetamine is a brain food, and less-so meth. People can do do cocaine every now and then (which itself is usually more *impure* and is more cardiotoxic than neurotoxic, though nuerotoxic). But with meth, people lose it much more often.
A several-year addiction to meth at these, when put into perspective, ridiculous doses via a potent route of administration, would be quite difficult to bounce back from. Hell, even prescribed stimualnts offer a great psychological dependence.
