Hello Possible causes may be Too much sun exposure resulting in dry lips and dark patches High intake of caffeinated beverages, such as, tea or coffee Consumption of hot food Biting and licking the lips constantly High levels of iron in the body Vitamin B deficiency Changes in hormone levels Allergic reactions to toothpaste and products applied on the lips Allergic reaction to lip medication...Consult nearest dermatologist they will help you further Regards