Dosages of dextroamphetamine that produce neurotoxicity are much higher than those used in human therapeutics. In animal experiments dosages are between 20-60 mg/kg injected. That would be 1.2-3.6 gr of injected d-amphetamine for humans. In animals, these dosages produce neurotoxicity but also death caused by hyperthermia of most specimens. And these effects do not appear in humans.

http://www.sciencedirect.com/science/article/pii/S0006899398008464

http://www.nature.com/npp/journal/v30/n11/full/1300771a.html

In science, proofs must be shown in positive. So, there are no data to suppose that d-amphetamine is neurotoxic at therapeutic dosages. Human dosages are 1000 times lower than neurotoxic dosages. There are no data also long time d-amphetamine exposure causes neurotoxic damage, and there is long time, intensive experience with amphetamine in humans for over a century.