Ah but
Only one person has picked up on a point that a lot of people are missing. Research on neurotoxic potential of a drug is performed on just the drug of abuse and not on "street" samples of drugs, which because of the lack of quality control, are far from meeting monograph from the British Pharmacopea (or those of other countries), and /navarone/ pointed out one of the worst culprits: methamphetamine. I'm not aware of any studies on the neurotoxicity of iodoephedrine and the aziridine that is also formed from the RP/I method, but one of the by-products is nephrotoxic, as there's a case of somebody from the bee site (who live in the same city as me) that fucked up his attempt at a meth synth, and ended up in ICU, being ventilated from tasting his own product, and the last my other half heard (she dealt with the call), his kidneys were shot, and they reckoned that he wasn't going to regain conciousness. All backed up by the fact that his flat had all the equip and chemicals he'd used, and some of his "product", when the boys in blue got involved. Admittedly, in one night he'd consumed more than the average person would get from using meth (unless they'd being doing it for quite a while).
Also, smoking ANYTHING is going to put polycyclic aromatic hydrocarbons into your system, and when they are metabolized, you end up with compounds that are DNA alkylators; don't matter what sort of cell it is then, alkylate one of the control genes so it has a copying fault, if it doesn't translate to the correct amino acid sequence for a protein that controls gene expression (DNA polymerase etc) and your immune system misses it, you've got a carcinoma - neurotoxic in a round about way if it happens to a neurone.
If they want to assess if a drug users are at risk of developing symptoms of neurotoxicity, surely they should be also be running trials with representitive samples of "street" drugs, or at least the major by-products/contaminants such as iodoephedrine.
Can't remember who brought up the "if it's neurotoxic for rats, then it will be in humans, just at a different dose" topic, but it doesn't always hold true. You can feed rats MPTP 'till it's coming out of their ears (not literally!) and they will not develop damage to the substantia nigra, or Parkinsons-like symptoms, but primates only need tiny, one time exposure to it and produces Parkinsons so severe that there isn't even a tremor at rest, the subject becomes "frozen", as with the opiate users in California who were unfortunate enough to get some in a faulty batch of MPPP.
