Enzyme extracted from papaya. According to Natural Medicines Comprehensive Database, “Topically, papaya latex can cause severe irritation and blisters. Topical use of papain can cause itching. Severe allergic reactions have been reported in sensitive individuals.” Other research has confirmed that papain is a strong allergen on skin (Source: Journal of Investigative Dermatology, February 2015, ePublication).
There’s one study showing it may be effective for exfoliation, but only in a pure concentration (Source: Archives of Dermatological Research, November 2001, pages 500–507).