A rare rash that shows up as lacy, white patches or red shiny bumps on the inside of the cheeks or tongue could be lichen planus. No one knows what causes it. Generally, mild lichen planus doesn't need any treatment. If it causes pain or ulcers, it can be treated with oral and topical medication. Oral lichen planus can be chronic and may increase the risk for oral cancer. Lichen planus can also affect skin, scalp, nails, and genitals.