The sprouts, or eyes, of a potato contain solanine, which can be toxic to humans in even small amounts; therefore, potatoes that have sprouted should be discarded and not eaten, as noted by the National Institute of Health's MedlinePlus. Potatoes that are green below the surface of the skin or have begun to rot can be poisonous as well, and should not be consumed.
As long as a potato is not green or has not begin to sprout, it is safe to eat. Some of the possible symptoms related to solanine include delirium, diarrhea, loss of sensation, abdominal pain and vomiting, as stated by the National Institute of Health's MedlinePlus. If someone experiences these symptoms after eating potatoes, it is best to see a physician immediately or contact emergency services. Try to keep the person from vomiting any further, if possible.