You've probably been told not to eat green potatoes because they are poison. Just how poisonous are they? The presence of chlorophyll in a potato's skin indicates an increased level of solanine, which at critical concentrations can cause horrible illness (vomiting, diarrhea, headaches, paralysis, and/or coma) and even death. It's happened more than you know. Smithsonian's Food & Think blog has a rundown of solanine poisoning cases spanning a century, with scary details. For example:
1952: According to the British Medical Journal, solanine poisoning is most common during times of food shortage. In the face of starvation, there have been accounts of large groups eating older potatoes with a higher concentration of the toxin. In North Korea during the war years of 1952-1953, entire communities were forced to eat rotting potatoes. In one area alone, 382 people were affected, of whom 52 were hospitalized and 22 died. The most severe cases died of heart failure within 24 hours of potato consumption. Some of the less severe symptoms included irregular pulses, enlargement of the heart, and blueing lips and ears. Those who displayed these ailments died within 5 or 10 days.
The amount of solanine in the average potato from your grocery store is small enough to be safe, but if potatoes are stored where light gets to them or they become old, the levels may be too high. So if a potato is green or past its prime, better throw it out.
(Image credit: Rasbak)