Most NSAIDs are chiral molecules: they exist under 2 configurations of non-superimposable mirror images which are termed enantiomers or optical isomers or optical antipodes. Direct or indirect (resolution) methods are used to separate this equal mixture of compounds. Some of the enantiomers of the NSAIDs are able to undergo chiral inversion from the inactive R(-) to the active S(+) form. The pharmacokinetics in terms of absorption, distribution, metabolism, protein binding and elimination may be different for the 2 enantiomers, leading to interindividual variability in clinical response and drug toxicity.