I am 23 years old female, I have a black longitudinal line on my big toe. It might have come before 3 or 4 years. I don't remember. I didn't even give much importance to it. But nowadays Isee here and there in net that it's something we shouldn't ignore, it might cause cancer and all. It's quite scary to see all that. So I have the se question? What is it? Do I really need to worry about it? I am brownish complexion person.