Answer
Dexedrine and Benzedrine are enantiomers of each other (one is $R$ chiral, the other $S$ chiral). Because chirality is extremely important for molecular interactions with enzymes/receptors in the body, it is likely that Dexedrine has the optimal chirality for the intended effect, while Benzedrine has a less than optimal chirality for intended effect, thereby requiring more of the drug to accomplish the same thing.
Because chirality of individual molecules does not affect physical properties like composition and melting point, the two drugs would still have identical physical properties.
Work Step by Step
Identify that the central C (connected to H, CH$_3$, NH$_2$, CH$_2$) is a chiral center since it has 4 unique substituents.
