Hi, The usual cause is repeated wetting and drying of the fingernails. This makes them dry and brittle. This is often worse in low humidity and in the winter (dry heat). The best treatment is to apply lotions containing alpha-hydroxy acids or lanolin containing lotions. very rarely are internal disease or vitamin deficiencies the reason (iron deficiency is the most common). Biotin (a vitamin) taken by mouth is beneficial in some people.