After sitting around for a while and perhaps rotting, potatoes contain a potent toxin called solanine. The substance is the plant’s natural defense against threats like illness and blight, but it also wards off creatures that eat potatoes. Like humans.
The most horrific cases of solanine poisoning happen not when someone grabs an old potato from the cupboard and says, “eh, what’s the worst that could happen?” Predictably enough, they happen during times of horrific famine, like North Korea at the beginning of the Korean War. That’s when a terrible combination occurs: people are foraging for any food they can get, including nasty moldy potatoes. They’re also malnourished, which amplifies the effects of the toxin.
There are cases were well-nourished humans have been poisoned, though. In the late ’70s, 78 schoolboys in London got sick from solanine poisoning traced to potatoes served at school. (Kids, this case is the exception: the lunch ladies are not trying to kill you.)
How can you avoid lurking ‘tater poison? If the outside of the tuber is green, don’t eat it. The green coating isn’t solanine, but is chlorophyll, which is an indication that the ‘tater has sat around in the light long enough to have high solanine levels.
Avoid this problem by storing your potatoes in a dark, cool, dry place. The “dark” part is key.