This study looks at using a special mouthwash to help prevent mouth sores (*stomatitis*) caused by a cancer drug called Dato-DXd. Dato-DXd is used for certain types of advanced breast and lung cancers. The study will include about 100 adults, divided into two groups: one for breast cancer and one for lung cancer. Participants must be at least 18 and have not used Dato-DXd before. They must also agree to use the dexamethasone mouthwash during Dato-DXd treatment. The study will last for about 18 months, with each person followed for at least 12 weeks after starting treatment. People with certain medical conditions, like active mouth infections or severe allergies, cannot join. Women who could have children must pass a pregnancy test before joining.

• Participants need to use a dexamethasone mouthwash during the study.
• The study will take place over approximately 18 months.
• Participants should not have taken Dato-DXd before joining.