I have an ulcer on my gums. it's been there for 3 days. i am a regular smoker with 5-6 cigarettes per day. what are the chances of a mouth cancer?

Rare at your age: An ulcer or canker sore is much more likely to be due to infection or irritation than to cancer at your age. But if you don't want cancer, avoid high risk behaviors or exposure to carcinogenic substances, like smoking/tobacco. If the sore does not go away in a few days or gets worse, see a doc: medical or dental.

Low chance: the chance of having are very low at your age. The ulceration may be a canker sore, infection, traumatic lesion. If this does not heal in 7-10 days the get it checked by a dentist.

Rare for your age: Unlikely. See oral surgeon or ENT doc if you have any of these symptoms: Persistent mouth sore, persistent mouth pain, a lump or thickening in the cheek, a white/red patch on the gums, tongue, tonsil, or lining of the mouth, a sore throat, hoarseness or feeling that something is caught in the throat that does not go away, difficulty swallowing/chewing.