We all get random little white marks or dots on our fingernails sometimes. Rest assured, those little "milk spots" are totally normal. (Usually.)
Typically, the marks—officially known as punctate leukonychia—are caused by trauma to the nail. It doesn’t have to be major trauma, like slamming your finger in a car door, though that would certainly qualify. Small things like excessive tapping and chewing can also cause the marks to appear. They form at the base of the nail bed and then grow out like the rest of the nail, so by the time you even notice a mark, you’ve likely forgotten what you did to cause it in the first place. (Unless you were a victim of the aforementioned car door. You’ll probably recall that.)
Occasionally, there’s a more nefarious reason for milk spots. Zinc deficiency can be a culprit, as can malaria, Hodgkin’s disease, and sickle cell anemia—but those causes are pretty rare. The authors of a very specialized book called The Nails in Disease note that the majority of punctate cases have no real cause.
However, there are a few types of leukonychia that should concern you, like leukonychia totalis. While it sounds like a Harry Potter spell, it’s really just a way to say that the entire nail has gone white. In that case, you want to consult a doctor, as completely white nails can herald more serious health problems, like kidney failure, heart disease, or diabetes.
Then there are Mees’ lines, which is when a white mark appears across the entire nail in a band. That’s a sign that you’ve been poisoned, most likely by arsenic or thallium. But, there's good news: if you’re alive to see the line appear, you’re probably in the clear.
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