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Sores in mouth can be due to poor oral hygiene
resulting in candid
infection;disease like fever
or GI infections;and deficiency of vitamins especially folic acid
.So get examined by a doctor to find the reason.Till then get started on tablets of vitamins and folic acid;for local pain you can apply zytee or Orasep or stomela gel which is meant for oral ulcers and it will help reduce the pain.