By the time a potato has grown roots, it's on its way to being bad: it may be soft and mealy, with wrinkles and patches of green on its skin, rather than firm and crisp, and you may not want to eat it. If the question is whether the potato is safe to eat – well, it depends.
Potatoes naturally have small amounts of solanine, a neurotoxin that in large amounts can cause a range of symptoms, from vomiting and diarrhea all the way to headache, hallucinations and shock. While healthy children and adults can typically tolerate the amount of solanine contained in a fresh potato, some sensitive individuals have reported joint pain. The roots, or sprouts, of a potato have a very high concentration of solanine. The solanine content in the potato itself increases dramatically as the potato is exposed to light. Green patches on or below the surface of the skin indicate high solanine content.
What to Do
Never, ever eat the roots, or sprouts, of a potato; cut them out, digging beneath the skin to get the whole root, and discard them. If the potato does not have mold or green skin you can eat it, but baking it isn't going to be the best culinary experience: peel it, then either boil and mash it or cut it up into a stew. If the potato shows patches of green, don't simply cut away the green skin and flesh and use it, advises MedlinePlus; throw the potato out.