The potato below has a green spot due to sunlight exposure. It is a result of chlorophyll formation. The green spot is called solanine. Keeping away potato under direct sunlight will prevent its formation.
Potato with solanine taste bitter but tasting it is forbidden. It causes health hazards and death. The compound is stable at boiling temperature. Removing the green spot is a safe practice and you can actually use the remaining. For safety reason, the best thing to do is throw away the monster potato.
Solanine is a glycoalkaloid poison found in species of the nightshade family, such as the potato. Naturally occurring in any part of the plant, including the leaves, fruit, and tubers. It is very toxic even in small quantities. It has fungicidal and pesticidal properties as plant’s natural defense.
Poisoning is is manifested by gastrointestinal and neurological disorders. Signs include nausea, diarrhea, vomiting, stomach cramps, burning of the throat, cardiac dysrhythmia, headache and dizziness. Hallucinations, loss of sensation, paralysis, fever, jaundice, dilated pupils and hypothermia. Ingestion in large quantities may cause death.