Phenolphthalein is a chemical compound often used as an indicator in acid-base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions. In medicine, phenolphtaleine was used as a laxative, but is now being removed from over-the-counter laxatives because of concerns over carcinogenicity. The mechanism of action of phenolphthalein is poorly understood. It is speculated that laxative effect of phenolphthalein in part to be mediated by the efflux of potassium from cells.