text: The antitubercular agent Ethambutol contains two constitutionally symmetrical stereogenic centers in its structure and exists in three stereoisomeric forms. An enantiomeric pair (S,S)- and (R,R)-ethambutol, along with the achiral stereoisomer called meso-form, it holds a diastereomeric relationship with the optically active stereoisomers. The activity of the drug resides in the (S,S)-enantiomer which is 500 and 12 fold more potent than the (R,R)-ethambutol and the meso-form. The drug had initially been introduced for clinical use as the racemate and was changed to the (S,S)-enantiomer, as a result of optic neuritis leading to blindness. Toxicity is related to both dose and duration of treatment.  All the three stereoisomers were almost equipotent with respect to side effects. Hence the use of S,S)-enantiomer greatly enhanced the risk/benefit ratio.