Use of (+)-(11R,2'S)-.alpha.-2-piperidinyl-2,8-bis(trifluoromethyl)-4-quinolineme thanol or a pharmaceutically acceptable salt thereof substantially free of its (-)-enantiomer in the manufacture of a medicament having reduced side-effect compared to the racemic (.+-.)-(R*,S*)-.alpha.-2-piperidinyl-2,8-bis(trifluoromethyl)-4-quinolinem ethanol for treating or preventing malaria in a subject. A method of treating or preventing malaria with reduced side-effects comprising administration of (+)-(11R,2'S)-.alpha.-2-piperidinyl-2,8-bis(trifluoromethyl)-4-quinolineme thanol or a pharmaceutically acceptable salt thereof substantially free of its (-)-enantiomer.