Everyone who wants it should have access. There's no reason production should be limited to the point you're having trouble accessing it. Illicit amphetamine has been cheap and readily available for decades.
This is largely my opinion (weakly held, however).
What's also common is ADHD in adults being self-medicated with things like caffeine and nicotine. I'm not saying that this will help their kid, but clearly we're ok with some things that are medicinal being uncontrolled.
This has already been tried. During the 1930s to 1950s, amphetamine was available over-the-counter under the trade name Benzedrine, but it was made prescription-only following rampant abuse. By then, it had garnered a significant reputation, and the folk proto-punk band "The Fugs" sang about it in their song "New Amphetamine Shriek".
The real issue with increasing legal access to speed is that you're never fully aware of the impairment that amphetamines cause, because they make the user feel cognitively-enhanced, regardless of reality. In contrast, with alcohol and cannabis, the user is generally well-aware that they are not safe to drive or operate heavy machinery.
I have known SO many people on prescribed amphetamines who either wrecked their cars or narrowly avoided doing so. When I was on it, I narrowly avoided rear-ending multiple different cars before it finally occurred to me that I probably shouldn't be driving. Until that happened, I felt like my driving was being made better by the amphetamine, and for repetitive aspects of driving it probably was. For reaction time and spatial awareness, amphetamine was impairing.
Think of the profound-seeming drivel-filled essay written by a college student cranking out an essay last-minute while high on Adderall and now imagine these people all over the place, driving large pickup trucks with similar misplaced confidence in their cognitive abilities.
Way worse than immediate cognative impairment - e.g. impaired driving is the slow mental degredation that comes from constant amphetamine use. It really, really fucks you up. Changes your personality, sense of humor, sexuality, etc. Over time. And usually youre having so much fun whjle youre high on it you dont even realize how fucked everything gets
