Phenolphthalein: A compound used as a laboratory reagent and acid-base indicator and also used in over-the-counter (OTC) laxative preparations.

Phenolphthalein may be a carcinogen (cancer-causing agent). Feeding of phenolphthalein caused cancer in multiple organs in multiple species of experimental animals. The US government in 2000 classified phenolphthalein as "reasonably anticipated to be a human carcinogen."