Oral cavity ulcer the 4th day, at the beginning a little pain, now not hurt at all, just the mouth has a strange feeling, take vitamin c apply on top, but instead from a small point, and become big white,... is this mouth ulcer more serious?

Mouth Ulcer: These lesions usually heal on their own within two weeks. For greater peace of mind and/or if it does not heal in a timely manner, please see your dentist for an exam.

Ulcer: Most oral lesions like the one you describe are self limiting, fully heal within 2 weeks of onset. Vitamin C (ascorbic acid?) can actually cause chemical burn of tissues. Please see your Dentist for evaluation and discussion of whether any additional treatment is required.