Methods and compositions are disclosed utilizing optically pure (−) pantoprazole for the treatment of ulcers in humans while substantially reducing the concomitant liability of adverse effects associated with the racemic mixture of pantoprazole. The optically pure (−) isomer is also useful for the treatment of gastroesophageal reflux. (−) Pantoprazole is an inhibitor of H+ release and is therefore useful in the treatment of other conditions related to gastric hypersecretion such as Zollinger-Ellison Syndrome.