Sorry for the confusing terminology. "Peripheral stimulation" means unwanted stimulation of the body itself, such as suppressed appetite, general jitteriness, and the like. Whereas the cerebral effects are the actual cognitive effects of improved attention/focus/alertness/scanning behavior, euphoria, and that kind of thing.

In general when taking amphetamines for ADHD-type symptoms, you want to maximize the cerebral stimulation while minimizing the peripheral stimulation, because the latter causes [most of] the unpleasant side effects like inability to eat/sleep. Note that some degree of peripheral stimulation is unavoidable regardless of whether one is taking pure d-methamphetamine or not, and also probably some amount of peripheral stimulation is desirable because ADHD is not just difficulty in maintaining focus/attention but also getting the kick in the ass to start tasks in the first place. But in my experience relying on the peripheral stimulation (which, for example, Adderall produces more of than Vyvanse) simply doesn't work long-term, and just makes appetite regulation and the like get totally out of whack.

If some of the illicit processing has reached a level of sufficient sophistication to enable removal of the l-enantiomer then I wonder how they go about it. Anyone know?

As this article shows, there's any number of ways of making MA but separating out it's racemic mixtures has always been considered hard for backyard-ers.

If this is now commonplace then it does seem to represent a somewhat of a seismic shift in the illicit drugs business model.

I don't know precisely but I remember it's something like bubbling d-tartaric acid through the racemic methamphetamine mixture to separate out the d-methamphetamine.

The overwhelming majority of US meth is cartel-sourced. They have massive superlabs and then transport kilos upon kilos across the border. That's how they have the sophistication to synthesize their own p2p, separate out the enantiomer, etc.

It's the inevitable result of the war on drugs. You make it near impossible to be a small time cook - due to the difficulty in acquiring precursors, acquiring space to work in where you won't get asked questions, etc - and soon the whole market is ran by mega cartels that operate as de-facto states.

Yeah, perhaps I should have thought about that solution before I asked the question it's sort of obvious when you think about it. Anyway, from yours and various comments from others, the consensus is that the d-tartaric acid method is the most likely approach. (That got me thinking, I've some supermarket tartaric in my kitchen with the herb containers. Until now I've not given much thought to its stereochemistry, but this story has almost got me enthusiastic enough to check whether it's chiral or meso. Polarizing filters off my camera ought to do the trick.)

Until this article, I suppose I'd had a rather naive view about such matters which I gleaned years ago (can't remember from where), that was that with any drug (or any other chemical for that matter) that had 100% 'd' or 'l' enantiomers then it had to be manufactured in (come from) a controlled industrial process due to the complexities of separation.

The trouble is I've only had a peripheral interest in the matter so I'm not up to date. Until this story I've been stuck with the old orthodoxy, which was that if law enforcement found 50/50% racemic mixtures then they had to come from illegal sources. Clearly, that's no longer the case, and it seems that it's been so for sometime, especially so with drugs made from pharmacy-grade precursors, pseudoephedrine, etc. - the pharmaceutical industry having done the hard work.

It also hadn't really dawned on me that mega cartels were employing professional chemists but it actually makes much sense given the mega dollars involved (I'd always assumed there were maverick and eccentric chemists around but never envisioned an illicit lab with a corporate structure run like a normal commercial chemical company).

Also, one could be forgiven for having missed the fact because for decades, news reports have painted a constant image about the dangers of street drugs due to their variable nature (not to mention their adulteration - being mixed and cut with dangerous substances in the midstream distribution chain). Coupled with the all-too-frequent TV images of makeshift labs in jungles with buckets of chemicals littered about everywhere one wonders how they produced drugs with any reasonable degree of purity at all. The last thing on my mind would have been that they'd have had the expertise or facilities to separate racemic mixtures. Clearly this preconceived notion is now outdated.

HN comments here have updated my thinking, they're very helpful and informative at times.