These are the first symptoms of a... See more in comments

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One of the most common early symptoms is a mouth sore that refuses to heal. Most minor ulcers disappear within a week or two, but a persistent sore lasting longer than two weeks should not be ignored. These sores may be painless at first, making them even easier to overlook.

Another warning sign is the appearance of unusual patches inside the mouth. White patches, known as leukoplakia, and red patches, called erythroplakia, can sometimes indicate abnormal cellular changes. While not every patch is cancerous, any persistent discoloration deserves professional evaluation.