 Mefloquines clinical utility has been impaired by its association with neuropsychiatric side effects. The pharmacological basis of mefloquines side effects is not known but two of the most reported hypotheses relate to its action on i the adenosine receptor and ii its effect on the cholinesterase enzyme. For both of these mechanisms, there is a significant stereoselective activity of the two enantiomers. In vitro studies show that the - isomer is 50-100 fold more potent towards adenosine receptors compared with the isomer. In addition, --mefloquine has considerably more anti-cholinesterase activity. It has therefore been hypothesised that -mefloquine may have a better central nervous system CNS safety profile compared with either the racemate or --mefloquine. 