Stereospecificity

Crucial to the specificity of ligand-receptor interactions is the molecular shape of the ligand. Most receptor binding sites are so specific, that stereochemistry of a ligand has strong impact on the binding capacity or receptor activation.
Example: verapamil (a calcium channel antagonist) is administered as

a racemic mixture (50% S-enantiomer and 50% R-enantiomer). See the graphic below.S-Verapamil is more potent in blocking the AV node, but it undergoes greater first-pass metabolism. The more active enantiomer also has a greater unbound fraction in the plasma.
A patient who suffers from severe constipation due to verapamil therapy could benefit from administration of a single enantiomer verapamil.