Disclosed is a method of preventing the development of mucositis in a subject undergoing radiation therapy or chemotherapy for a disease in need thereof comprising administering an effective amount of a mammalian target of rapamycin (mTOR) inhibitor, such as rapamycin, to the subject. Further disclosed is a method of increasing the lifespan of a normal oral keratinocyte and/or reducing oxidative stress in a normal epithelial cell, wherein the method comprises administering an effective amount of an mTOR inhibitor to a subject undergoing radiation therapy or chemotherapy for a disease in need thereof.