I think it might be helpful if I share a little bit about my first-hand experiences with the substances mentioned in this thread. As most of you know, Heroin/Morphine represented 90% of my hard drug usage. I also would drink a fair bit back in the day. That doesn't mean I haven''t taken every other kind of drug out there though.
I was considered a very intelligent student, but I also had zero aptitude for the logistics involved with homework, punctuality etc. When teachers were putting work in front of me, it was easy. When I had to manage time and goals myself, I completely hit a wall. This is what led to my first foray with prescription stimulants. The issue was, at this time, I was also beginning to experiment with hard drugs like Oxycodone/Heroin etc. Amphetamines were just another drug to abuse for me. I thus never received consistent benefit with the medication.
I would go on to abuse Amphetamines recreationally, intermittently throughout my adult life. There were a lot of Amphetamines available in Cambodia, either Ya-Ba or shards of Meth. In Iraq you could get a script for Methylphenidate pretty easily, but you could also buy Captagon (fenethylline) just as easily. I would use these mostly to wake myself up if I was too sedated from Opioids. I had never really attempted to use Amphetamines in any manner that could be considered "responsible". Likewise, I assumed that I was probably incapable of using them in such a way.
After I had been off Heroin for a few years, I somehow came to the conclusion that I should try a stimulant prescription again. This was a pretty risky proposition and in retrospect, I don't know what had gotten into me thinking this would be okay. I made a decision that I would trust my medication with another person to see how I would do with it. This is a practice I cannot recommend enough for folks struggling with medication compliance.
I was pretty blown away when I took over the prescription after 3 months. I was actually able to take it as prescribed. I had seen how beneficial it was for me to have the medication and my matured mind did not want to sacrifice these benefits just to get really high. In short, the positive effects on my professional/personal life was getting me higher than the drugs would, if that makes sense.
I have used Methamphetamine. See, Methamphetamine didn't really exist in New England outside of the insular community of Gay men who used it. It came around for the first time maybe 10 years ago. My point is, it was not a feature of the drugs landscape in my childhood -> age 24-45.
You could say Methamphetamine is, apples to apples, just a more potent Amphetamine. If one is to adjust the dosage accordingly, they should be essentially interchangeable. In my experience, that is overly simplistic. See, I have used Methamphetamine on whim for the same reasons I might buy a couple of Adderall tablets; I needed to work an overnight shift or something like that. Well, I did this a couple of times and found I could never stop before I went over a line I didn't want to go over.
I would go into these experiences absolutely only wanting to be "more awake" for a reasonable amount of time, say, for the duration of my shift. Instead, I would end up staying awake for 3 days. I would end up using a bit too much to be able to go to sleep, right, then l would have no choice but to use more Methamphetamine to stay awake for that shift and by day 3, I'd feel like a hollowed out husk/robot. I've always heard stories of folks staying up for a week, two weeks... three weeks? Fuck, I can't even comprehend how my body would feel. as 3 days awake seemed like it was totally devastating.
I eventually came to terms with the fact that I could not use Methamphetamine in a reasonable way. It was either I would end up on a binge or I wouldn't be doing it at all. This is the relationship I have with the majority of psychoactive substances to this day. I can take Methadone, Gabapentin/Pregabalin, Cannabis and my prescription Lisdexamfetamine (Vyvanse), but the majority of substances derail me from being functional.
So I was surprised when I found I was able to control my usage with the Lisdexamfetamine. When I started taking it, I was very apprehensive and cautious. For whatever reason, I've never just taken my whole prescription in a few days. I've never completely lost control. I'm not sure if this is maturity or if it's the result of harmonious biochemistry; I see the benefits I can gain from the medication and I would rather have those benefits than to get wicked high for a few days.
The pharmaceutical industry is known to be pretty unscrupulous. Still, I think it's worth noting, Methamphetamine was once prescribed alongside non-methylated varieties. Methedrine was a really big brand name for some time. If you're a nerd like me and you enjoy reading about the history of pharmaceuticals, you will notice that throughout the 50's-early 70's Methamphetamine was often singled out as especially addictive/dangerous. You also hear of users seeking out Methedrine over the non-methylated stuff.
This time period was noted as an Amphetamine-epidemic. These drugs were given out like candy until the 1973 Dangerous Drugs Act, which caused Amphetamine production to drop by 90% virtually overnight. I believe there is ultimately a good reason why Methamphetamine is not prescribed in favor of Amphetamine.
My final point, you can't ever minimize the importance of having pre-portioned, standardized dosage units for drugs. Whether a drug is in the form of a pill or a powder, it shouldn't make a difference if we can utilize basic math; at least that's what we tell ourselves. In practice, having drugs in the form of loose powder is inherently more dangerous in terms of the potential for abuse. Not only is it more difficult to measure dosages, the psychological characteristics of addiction mean it's always easy to add just a little bit more and tell yourself it's nothing.
If you had pills containing a standardized amount of pure Methamphetamine, I'd say it's certainly possible to use it in the same manner in which Amphetamine is used. However, I think its addictive potential is always going to be greater than that of Amphetamine, making it the worse choice for functionality, especially in the long-term.