Interestingly enough Mansal from Nootropedia just talked about this very topic, with a study from European Neuropsychopharmacology journal showing longterm or chronic use of amphetamines have been shown to decrease NFG (Nerve-growth Factor) and BDNF (Brain-derived Neurotrophic Factor) in rats. Adderall is an amphetamine treatment for ADHD, and also applies to Vyvanse, Dextroamphetamine, and other drugs.

This study demonstrates the clear risks that the Nootropics community sees with regular and longterm usage of such drugs, you will definitely get short term benefits on concentration, focus, productivity, and all the associated decreases in your specific ADHD symptoms, but likely at a longterm cost to these very same faculties in your brain as NFG and BDNF play central roles here! As Mansal notes; "If Adderall is absolutely necessary to function, then maybe it is worth the trade off. I presume there are millions for whom this is a BAD trade, though."

The study:
Chronic amphetamine treatment reduces NGF and BDNF in the rat brain

Abstract
Amphetamines (methamphetamine and d-amphetamine) are dopaminergic and noradrenergic agonists and are highly addictive drugs with neurotoxic effect on the brain. In human subjects, it has also been observed that amphetamine causes psychosis resembling positive symptoms of schizophrenia. Neurotrophins are molecules involved in neuronal survival and plasticity and protect neurons against (BDNF) are the most abundant neurotrophins in the central nervous system (CNS) and are important survival factors for cholinergic and dopaminergic neurons. Interestingly, it has been proposed that deficits in the production or utilization of neurotrophins participate in the pathogenesis of schizophrenia. In this study in order to investigate the mechanism of amphetamine-induced neurotoxicity and further elucidate the role of neurotrophins in the pathogenesis of schizophrenia we administered intraperitoneally d-amphetamine for 8 days to rats and measured the levels of neurotrophins NGF and BDNF in selected brain regions by ELISA. Amphetamine reduced NGF levels in the hippocampus, occipital cortex and hypothalamus and of BDNF in the occipital cortex and hypothalamus. Thus the present data indicate that chronic amphetamine can reduce the levels of NGF and BDNF in selected brain regions. This reduction may account for some of the effects of amphetamine in the CNS neurons and provides evidences for the role of neurotrophins in schizophrenia.

Keywords: Amphetamine, NGF, BDNF, Rat brain, Schizophrenia
https://www.europeanneuropsychopharmacology.com/article/S0924-977X(07)00061-2/fulltext