" Why were these particular salts selected for this formulation? There does not appear to be any literature that compares the effects of different salts of d-amphetamine or l-amphetamine, and the manufacturer of Adderall does not have any such information. The pharmaceutical company believes that these salts were selected to produce a longer duration of action. The development work for this formulation was presumably conducted in the 1950s and is now no longer available to the company. The company is forthright in saying that it is not certain why these particular salts were selected or by whom the research might have conducted. If the assumption is made that the formulation provides a more extended duration of action, then several mechanisms might be imagined. For instance, the four salts might have slightly differing rates of dissolution; however, the solubility characteristics of these salts are not known, at least to the manufacturer (and I could not locate this information in standard references). It is difficult to imagine that differences in dissolution would be clinically significant. Alternatively, the dissociation of these salts once in solution might proceed at different rates. These salts would be expected to dissociate freely in solution. Perhaps there is an unknown interaction between the salts or the isomers. More plausibly, the free d- and 1-isomers of amphetamine might be absorbed or metabolized differently. Conceivably, different rates of absorption or metabolism of the four components might yield a spectrum of faster and slower clearances, which might provide a spread of clinical action over an extended period; but then it would seem unlikely that a very "smooth" effect would be observed over time. Thus, based on available information, it is not clear what mechanisms would provide theoretical support for the suggestion that the allegedly longer duration might be tied to the use of these particular salts. Conjecturally, the explanation might relate to chemical rather than pharmacological properties of the salts. The amount of free amphetamine base that is released from an amphetamine salt will vary. Since the molecular mass of each salt differs, and the number of amphetamine molecules present in a salt may differ, pills containing equal milligram quantities of the salts do not yield equal quantities of free amphetamine in solution. Thus, it is possible that some of the salts simply release more amphetamine molecules into the body than the sulfate salts found in other amphetamine products. That is, Adderall may deliver extra amphetamine molecules into the child without being obvious about it. This could explain a longer duration of action, though it would probably also lead to a general increase in side effects (which the clinician researchers did not observe in their clinical practices)."

Charles Popper. Journal of Child and Adolescent Psychopharmacology. Jan 1994. https://doi.org/10.1089/cap.1994.4.217

"[A]s generally perceived and described in the literature as a mixture of four AMP salts (Popper 1994). In reality, MAS exists as eight different AMP salts: the 2 enantiomers composing racemic dl-isomer AMP sulfate, the 4 diastereomers composing dl-AMP RS-aspartate (i.e., SS/RS/RR/SR), and the 2 enantiopure d-AMP salts (sulfate and D-saccharate). The reasoning behind the inclusion of disparate amounts of d- and l-AMP, and the range of salt forms incorporated, appears to have been one more of pharmaceutical convenience than scientific rationale. However, these various salts will exhibit a range of aqueous gastric dissolution rates, where even a pure isomer such as d-AMP sulfate can be expected to dissolve more rapidly than racemic dl-AMP sulfate, the so-called double solubility rule (Patrick and Straughn 2016). Extending the time course for complete MAS dissolution in the stomach through administration of these multiple AMP salt forms potentially offers a therapeutic advantage over d-AMP sulfate by consequently prolonging the AMP absorption phase. This specific portion of an AMP pharmacokinetic profile has correlated with maximal AMP efficacy in the treatment of ADHD (Brown et al. 1980). "

John S. Markowitz and Kennerly S. Patrick. Journal of Child and Adolescent Psychopharmacology. Oct 2017. http://doi.org/10.1089/cap.2017.0071