The effects of chemotherapy on dividing cells are rapid. Cancer cells such as bone marrows, the mouth lining, and the gastrointestinal tract are fast in dividing cells. Chemotherapy also causes mouth sores. Minor sores heal in one to two weeks while severe ulcers could easily be infected with bacteria or the most common yeast found in the mouth. There are, however, medications that help prevent sores. To know more about Preventative dentistry, you can ask your dentist about it.