One in four of all therapeutic agents are marketed and administered to man as mixtures. These agents are not drug combinations in the accepted meaning of the term, i.e. two or more coformulated therapeutic agents, but combinations of isomeric substances the biological activity of which may well reside predominantly in one isomer. The majority of these mixed formulations arise due to the use of racemic mixtures of synthetic chiral drugs and less frequently to mixtures of diastereoisomers. Over the last ten years there has been considerable interest in the area of drug chirality as a result of recent advances in the stereoselective synthesis and stereospecific analysis of chiral molecules. As a result of these advances and the realization of the significance of the pharmacodynamic and pharmacokinetic differences between the enantiomers of chiral drugs there has been increasing concern over the use of racemates, and other stereoisomeric mixtures, in therapeutics. The use of such mixtures may present problems particularly if the adverse effects, or toxicity of the drug is associated with the less active, or inactive isomer, or does not show stereoselectivity. Many authors regard racemates as “compounds which contain 50% impurity” and that their use is “polypharmacy” with the proportions of the materials being dictated by chemical rather than pharmacological or therapeutic criteria. As a result of these concerns drug stereochemistry has become an important consideration for both the pharmaceutical industry and the major drug regulatory authorities.