Combining D-amphetamine  and methylphenidate: Effects on Amphetamine Neurotoxicity

Some animal models have shown neurotoxic effects of dextroamphetamine at very high doses. However, the dosages prescribed for humans are typically below the neurotoxic threshold. The neurotoxic potential of methylphenidate, an amphetamine derivative, is even lower and considered insignificant. Therefore, there is no need for concern about neurotoxicity when taking these drugs at the prescribed dosage via oral route.

So, combining D-amphetamine (Dexedrine® ) and methylphenidate (Ritalin®) would not have any effect on neurotoxicity as it is unlikely to occur at therapeutic doses. However, combining the two drugs can increase the risk of adverse effects, and therefore, it is not recommended. 

http://www.ncbi.nlm.nih.gov/pubmed/9365033 http://www.ncbi.nlm.nih.gov/pubmed/22289608
http://www.ncbi.nlm.nih.gov/pubmed/2440058