The behavior of the chiral twins depends mainly on the nature of the environment (achiral/chiral) in which they are present. An achiral environment does not differentiate the molecular twins whereas a chiral environment does distinguish the left-handed version from the right-handed version. Human body, a classic bio-environment, is inherently handed as it is filled with chiral discriminators like amino acids, enzymes, carbohydrates, lipids, nucleic acids, etc. Hence when a racemic therapeutic gets exposed to biological system the component enantiomers will be acted upon stereoselectively.  For drugs, chiral discrimination can take place either in the pharmacokinetic or pharmacodynamic phase.