L-meth does basically nothing when paired with meth. L-meth is SLIGHTLY more selective of norepinephrine than D-meth, and has very much less affinity for dopamine than D-meth. But when paired together they both do in fact increase synaptic cleft concentrations of monoamines; dopamine, serotonin, and norepinephrine.
L-meth isn't slightly more selective for NE than d-meth. It is tremendously more selective for NE, by an order of magnitude.
Make no mistake, 90:10 d-meth:l-meth is a potent, highly abusable stimulant. Functionally speaking, it isnt much different than d-meth. What makes it more likely to produce psychosis than the old pseudoephedrine based d-meth isn't so much pharmacodynamic differences but a function of economics: this new cartel meth is a fraction of the price, so users can take more, and sustain high dose daily habits far easier. That is what Sam Quinones missed. Instead he invented the P2P meth boogeyman.
Trace amounts of L-meth don't destabilize the subjective high of D-meth.
It doesn't destabilize it, but it definitely does alter it. For those who have tried enantiomerically pure, pseudoephedrine based crystal meth, the difference is quite notable.
Beyond pharmacodymanics, probably of greater importance is that pharmacokinetically, l-methamphetamine, even at fairly low proportions, competes for absorption, distribution, and metabolic pathways, which can lower the effective concentration of d-methamphetamine in the brain, especially the all important time pharmacokinetic figure of time to peak plasma (which is a major component in the rewarding effect). The ultimate result is that the dopaminergic activity is not as pronounced as it would be with pure d-methamphetamine.
Also both enantiomers are metabolized by similar enzymatic pathways, so the presence of l-methamphetamine will slightly inhibit metabolism of d-methamphetamine due to competition for these enzymes, making it last a bit longer (similar to adderall, whereby l-amp alters d-amp pharmacokinetics compared to dextromethamphetamine alone). I suppose one could argue this could increase the risk of psychosis and neurotoxicity, but I'd say the difference is minor. The bigger difference is the high.
So in a sense, this new meth is slightly less reinforcing than pure d-meth. I know a long-term tweaker who actually quit because he didn't like the high of this stuff. For a new user smoking this new meth versus enantiomerically pure d-meth, they might be less impressed.
I remember trying this new breed of meth for the first time, when smoked i thought, "damn this stuff is not so great". Orally however, it is quite similar. This is because of the pharmacokinetic issue is more evened out.
So yeah, I agree Sam Quinones got it wrong about the P2P derived meth being somehow more toxic or addictive than pseudoephedrine based meth. But l-meth makes a difference. Intravenously it is probably is a bit worse for you given the potent local vasoconstriction that l-meth causes, and might be slightly more psychosis inducing since it hangs around longer, but the main reason people are losing their shit on it is is because how dirt cheap it has become, and how widely available it is.
I'm not a fan of meth, even the best d-meth, but there is a difference with this new stuff. And difference isn't the difference Sam Quinones described. The subjective difference is more a note for drug connoisseurs. The real issue with this new shit is that its dirt cheap and everywhere.