The question of desoxyn (meth) being in the clinic for narcolepsy is that it MUST come on quicker (methylated being more lipid soluble) and last longer (better protected from MAO) than AMP. You can take AMP and wait the 15-20 mins or so to make it to work or school, but when you are narcoleptic and you feel an attack coming on when driving in heavy traffic and can't easily pull off the road for a nap, a quick acting jolt that will last awhile and be strong enough, is what you need.
Eh, that seems like a completely superfluous theory as it would be just as easy to produce a dexamphetamine sublingual tabletor nasal spray if speed of onset was that important. Considering that given a slightly higher dose, dexamphetamine is just as decent a locomotor stimulant, only it doesn't have the euphoria of d-methamphetamine.
On top of everything else, sec amines are more basic than primary and increasing polarity of a molecule, as a rule, reduces lipid solubility unless it's a long chain alkyl group that can form a micelle like structure
My biggest concern is that such dopaminergic compounds as AMP and meth, are structurally related to MPTP.
Other than containing an aromatic ring and an amino function, they're not that much like each other. Amphetamines are at their core a Ph-C-C-N and unconstrained, This is vastly different to the Ph-C-C-C-N of MPTP, which is conformationally restrained due to the heterocyclic ring. Add to this the ease with which the piperidine ring is easily converted to a stable aromatic pyridinium species by MAO-B compared with the elimination of the amino/methylamino group in amphetamine/meth and there's very little they have in common.
I've certainly never heard of dextroamphetamine being a carcinogen.
The secondary amine of meth can undergo a reaction with nitrites found in meat preservatives to produce a nitrosoamine; under the same conditions a primary amine like amphetamine is converted to an alcohol, which isn't toxic (tertiary amines are unreactive in the presence of nitrites). Now nitrosoamines are well known to be carginogenic, which is yet another black mark against methamphetamine.
Finally, methampheamine exerts it's neurotoxic effects due to the depletion of neurotransmitters dopamine & 5HT from the neurones. Eventually this leads to a situation where 5HT neurone will take up synaptic dopamine & this is what buggers them rather than some quaternary aromatic compound like MPP+
PS Much more unpleasant, (neuro)toxic drugs are used every day to treat a variety of conditions, it's a risk benefit assessment. You only have to look at something like the vinca alkaloid, vincristine for a really nasty example.
Here's a much bigger list of neurotoxic toxic drugs that have acceptable risk factor considering what they're used for treating