Many biologically active synthetic drugs contain chiral centers, although they are used as racemic mixtures. Enantiomers are hard to distinguish in the chemical laboratory but are readily discriminated in the body and differ in their biological activities and disposition. The pharmacokinetic profiles of enantiomers can be variable, especially for drugs with a first-pass effect and enantioselective pharmacokinetic monitoring should be carried out. Ultimately, whether to exploit a racemate or a single enantiomer in therapy is a multi-faceted decision to which drug disposition data have important contributions to make.