Letosteine is an expectorant. It dissolves bronchial mucus and reduces respiratory inflammation symptoms. Letosteine and ambroxol hydrochloride provided equivalent efficacy and safety in the treatment of sputum thickening and expectoration difficulty due to either acute or chronic respiratory diseases. Treatment with letosteine leads to a significant increase in the rate of regression of thoracic symptomatology and a faster, more substantial reduction in fever in children suffering from acute bronchitis. Letosteine had antioxidant activity. It may have a therapeutic application in preventing oxidative tissue injury damage induced by the respiratory burst. In a series of 37 patients, two experienced gastralgia and vomiting, severe enough to stop therapy. In another series of 40 patients, five had gastralgia and nausea, requiring withdrawal of therapy in three.