Different stereoisomers of drugs have different pharmacological activities (they act in different ways in your body).
Pharmalogical activity is dependant on drug interaction with proteins (receptors or enzymes), nucleic acids (DNA/RNA), and biomembranes (phospholipids/glycophospholipids).
All of the macromolecules mentioned above are built from chiral monomers so the 3D structure of a chiral drug determines how it binds to the chiral receptor and with what affinity.
This means that all potential new drugs must have all their isomers individually characterised.
