| • Idiopathic keratosis | • Infections |
| • Carcinoma | Candidiasis |
| • Skin grafts | Hairy leucoplakia |
| • Physical or chemical | Syphilitic keratosis |
| Friction | Papillomas (some) |
| Burns | • Mucocutaneous disease |
| Chemicals | Lichen planus |
| Tobacco | Lupus erythematosus |
| Snuff | Inherited lesions (such as white sponge naevus) |
| Sanguinarine | |
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