Papaya (Carica papaya) is a tropical tree. Its ripe fruit is considered safe, but unripe papaya fruit contains papain and can damage the esophagus. Unripe papaya contains a chemical called papain. Papain breaks down proteins, carbohydrates, and fats. But papain is changed in the stomach, so it's not clear if it's effective as medicine when taken by mouth. Papaya also contains a chemical called carpain, which seems to be able to kill certain parasites and might affect the central nervous system.