Long horizontal lines on nails may be caused by nail malnutrition, nail fungus disease, other skin diseases and other reasons. 1. Nail malnutrition: Nails with long horizontal stripes may be caused by nail malnutrition, such as long-term partiality, picky eating, etc. leading to vitamin and trace element deficiencies, or nails are subjected to long-term, chronic chemical stimulation, such as disinfectants, detergents, nail art products, and so on. 2. Nail fungus disease: long horizontal lines on the nails may be caused by nail fungus disease, which can lead to horizontal lines on the nails and also change the color of the nails. 3. Other skin diseases: Certain skin diseases can also damage the nails, such as psoriasis, lichen planus, etc. These diseases will not only damage the skin, but also make the nails grow horizontal lines. In serious cases, it is recommended to actively seek medical attention to identify the cause of the disease and follow the doctor’s instructions for targeted treatment.