Cheilosis is the cracking and inflammation at the corners of the mouth, classically caused by riboflavin, vitamin B2, deficiency, though iron deficiency and ill-fitting dentures can contribute. The fissures are painful, slow to heal because the mouth is constantly in motion, and prone to secondary fungal infection. It appears in the textbook cluster of B-vitamin deficiency signs alongside glossitis and seborrhoeic skin changes. Treatment addresses the cause: riboflavin-rich foods such as dairy, eggs, and meat, or supplements where diet is inadequate, plus antifungal cream if infection has set in. It is a visible, treatable marker of undernutrition.