Here is the best assumption I can make from all the research I have done for humans as most of the data is from studies on rodents and even primates.
Pure Dexmethamphetamine taken IV as in the above mentioned models:
PRN low doses (well under 0.5 mg/kg) can be surprisingly and notably beneficial, as you will see in the link below this editorial.
Daily doses of up to 0.5 mg/kg are considered safe but chronic regular daily doses of 1.0 mg/kg may well eventually cause toxicity over time.
Doses above 2.0 mg/kg are probably somewhat toxic at any frequency.
Street Meth may contain a myriad of cuts and dangerous additives or impurities as well as being racemic (a variable ratio of the l-meth and d-meth stereo-isomers) so it's much harder to predict it's true pure weight, safety profile and psychostimulatory effects. Some contaminants may stay in your tissue forever so it's really smart to know your source as well as possible.
Washing your product in pure Acetone can help but some cuts need isotropyl alcohol and who knows what else ?!
* the FDA recommends doses from 5 mg up to 25 mg of Desoxyn (oral - a lower bioavailability of approx. 70% - pharmaceutical Meth to treat severe obesity / eating disorder and severe ADHD). Doses up to 40 mg are possible though but most doses would be not much at all, all things considered.
Note: one paper states that the MAXIMUM THERAPEUTIC dose of both d-amph and d-meth is 60 mg. This can be found in:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6135110/#:~:text=The%20net%20result%20of%20the,striatum%2C%20which%20results%20in%20excitotoxicity.
In advanced cases the nerve terminal damage can be permanent leading to not only a trashed reward (no pleasure when sober) system and also a faulty impulsivity control system - just to make it near impossible to stop - eventually causing even Parkinson's disease and Dementia.
Of couse, NEVER FORGET that there is also the possibility of death from organ failure too !
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The duality:
* the good
https://pubmed.ncbi.nlm.nih.gov/21635908/* the bad
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7457172/* the best
Protective Effect of Low Dose of Methamphetamine on The Amount of Extracellular Glutamine in Primary Fetal Human Astrocytes Induced by Amyloid Beta:
https://pubmed.ncbi.nlm.nih.gov/35451579/Traumatic Brain Injury and stroke benefits:
https://www.sciencedirect.com/science/article/pii/S0278584615000469