Potatoes contain glycoalkaloids such as solanine, toxic compounds which are usually more prevalent in the wild potato varieties. Cooking at high temperatures usually removes the toxin, although headaches, diarrhea, cramps and in severe cases death can occur in rare cases.

This is why you're told to keep potatoes away from light. It's to prevent higher concentrations of solanin from developing. So don't eat potatoes after they have turned green!