AbstractThere is a great deal of current interest in the role and importance of chirality in the development of new drugs, but little attention is being paid to the stereochemistry of older drugs. Indeed, many older chiral drugs were introduced without adequate information on their stereochemical identity or composition. We have examined one such drug, the antihypertensive diuretic agent cyclothiazide. Standard sources of drug information and the research literature do not provide data on the stereochemical composition of clinically used cyclothiazide, although scattered reports indicate that the drug may consist of “several stereoisomers.” Inspection of the chemical structure of the drug, 6‐chloro‐3,4‐dihydro‐3‐(5‐norbornen‐2‐yl)‐2H‐l,2,4‐benzothiadiazine‐7‐sulfonamide 1,1‐dioxide, shows that it can exist as eight stereoisomers that may form four racemates. Using synthesis, fast‐atom‐bombardment mass spectrometry, gas–liquid chromatography, chiral and nonchiral high‐performance liquid chromatography, and nuclear magnetic resonance spectroscopy, we determined that pharmaceutical cyclothiazide is in fact a mixture of the eight stereoisomers in the form of the four racemates. The two racemates withendoconfiguration at the norbornene moiety predominate over theexoracemates, and small but significant differences in isomer distribution between different batches of the drug were observed. We urge that in studies of older drugs t