There can be several causes of lip cracks, including:

1. Dehydration: Lack of hydration can cause the lips to become dry and crack.
2. Weather: Exposure to cold and dry weather can cause the lips to become dry and crack.
3. Sun exposure: Overexposure to the sun can cause the lips to become dry and crack.
4. Licking lips: Licking your lips frequently can remove the natural oils from the lips and cause them to become dry and crack.
5. Allergies: Certain allergies, such as allergic reactions to lip balms or toothpaste, can cause the lips to become dry and crack.
6. Medical conditions: Certain medical conditions such as hypothyroidism, Sjögren's syndrome, and diabetes can cause the lips to become dry and crack.
7. Nutrient deficiencies: Deficiencies in certain nutrients such as vitamin B and iron can cause the lips to become dry and crack.

If you are experiencing persistent lip cracks, it's best to consult a doctor or dermatologist to determine the underlying cause and appropriate treatment.

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