In this video, Mark Yoder (N.P., Oncology) gives an overview of mouth sores, and some tips for dealing with this common treatment side effect. Mouth sores most commonly occur due to the cells in the mouth breaking down as a result of cancer treatment. This side effect can be very painful and trigger other issues such as lack of appetite and weight loss. There are remedies that may help, so it is important to contact your physician as soon as possible to discuss your options.