What causes white spots underneath your fingernails?

White Spots Nails: Difficult to answer without seeing these. It sounds as if it might be a harmless superficial fungous infection. Or, perhaps, from a nail cosmetic you may be using. See a dermatologist if you are worried about them.

Many causes: Milk spots on nails are usually harmless & related to nail trauma (such as biting nails). Other causes can be Iron, zinc or protein deficiency, lead or arsenic poisoning, renal failure, cirrhosis, pneumonia, heart disease, psoriasis or marked psychological stress.